A similar issue can be also observed in RadTreeView:

Workaround RadGridView: 
private void RadGridView1_CellEditorInitialized(object sender, GridViewCellEventArgs e)
{
    RadTextBoxEditor editor = e.ActiveEditor as RadTextBoxEditor;
    if (editor != null)
    {
        editor.EditorElement.MinSize = new Size(0, 30);
    }
}

Workaround RadTreeView:
 
private void RadTreeView1_EditorInitialized(object sender, TreeNodeEditorInitializedEventArgs e)
{
    TreeViewTextBoxEditor editor = e.Editor as TreeViewTextBoxEditor;
    if (editor != null)
    {
        ((BaseTextBoxEditorElement)editor.EditorElement).TextBoxItem.TextBoxControl.MinimumSize = new Size(0, 24);
    }
}

In 125% the RadTextBox in the Fluent theme keeps shrinking:

Workaround
public RadForm1()
{
    InitializeComponent();

    SizeF dpi = NativeMethods.GetMonitorDpi(Screen.PrimaryScreen, NativeMethods.DpiType.Effective);
    this.radTextBox1.MinimumSize = new Size(0, (int)(24 * dpi.Height));
}

Use attached to reproduce.

Workaround:
private void RadGridView1_ColumnChooserCreated(object sender, Telerik.WinControls.UI.ColumnChooserCreatedEventArgs e)
{
    var columnChooser = e.ColumnChooser as GridViewColumnChooser;
    columnChooser.Shown -= ColumnChooser_Shown;
    columnChooser.Shown += ColumnChooser_Shown;
  
}

private void ColumnChooser_Shown(object sender, EventArgs e)
{
    var columnChooser = sender as GridViewColumnChooser;
    var factor = this.FormElement.DpiScaleFactor;
    columnChooser.Scale(factor);
}

Use attached to reproduce.

Workaround:

Use CellFormatting instead of RowFormating. 

To reproduce:
private void radButton1_Click(object sender, EventArgs e)
{
    radGridView1.MasterView.TableSearchRow.SelectNextSearchResult();

}
Check the index in the search textbox, it is not updated. 

Workaround:
var cell = radGridView1.TableElement.FindDescendant<GridSearchCellElement>();
if (cell != null)
{
    cell.FindNextButton.PerformClick();
}

To reproduce:
Use the following code
private void RadGridView1_UserAddingRow(object sender, GridViewRowCancelEventArgs e)
{
    radGridView1.MasterView.TableAddNewRow.CancelAddNewRow();
    radGridView1.MasterView.ViewTemplate.DataView.CurrentItem.IsCurrent = false;
}

Place the new row at the bottom. While in edit mode press tab to fill until an exception is thrown.

Workaround:
BaseGridBehavior gridBehavior = radGridView1.GridBehavior as BaseGridBehavior;
gridBehavior.UnregisterBehavior(typeof(GridViewNewRowInfo));
gridBehavior.RegisterBehavior(typeof(GridViewNewRowInfo), new CustomGridRowBehavior());


internal class CustomGridRowBehavior : GridNewRowBehavior
{
    protected override bool ProcessTabKey(KeyEventArgs keys)
    {
        bool newRowIsInEditMode = this.IsInEditMode && this.GridViewElement.CurrentRow is GridViewNewRowInfo;

        if ( newRowIsInEditMode  && this.GridViewElement.Navigator.IsLastEditableColumn(this.GridViewElement.CurrentColumn))              
        {
            this.GridViewElement.Navigator.SelectFirstColumn();

            while (this.GridViewElement.CurrentColumn.ReadOnly && this.GridViewElement.Navigator.SelectNextColumn())
            { }
            return false;
        }
        return base.ProcessTabKey(keys);
    }
}

Use attached to reproduce.

Workaround:

DragDropService.AllowAutoScrollColumnsWhileDragging = false;
DragDropService.AllowAutoScrollRowsWhileDragging = false;

To reproduce: add a grid and a button and use the following code snippet:

[WORKAROUND: call Application.DoEvent at the beginning of the BindGrid method before rebinding]

   public partial class Form1 : Form
    {
        DataTable dtLeg;
        DataTable dtComp;
        DataTable dtClass;
        Int32 intClickCount = 0;
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            try
            {
                BindTables();
            }
            catch(Exception)
            {
                throw;
            }


        }

        private void BindTables()
        {
            try
            {
                dtLeg = new DataTable();
                dtComp = new DataTable();
                dtClass = new DataTable();

                Random rand1 = new Random();
                Random rand2 = new Random();
                Random rand3 = new Random();
                Random rand4 = new Random();

                int cNo = rand1.Next(2000, 10000);
                int dID = rand2.Next(2000, 10000);
                int dDetID = rand3.Next(2000, 10000);
                int depCode = rand4.Next(1000, 8000);
                string cCls = "Leg MIAGCM " + depCode.ToString();


                dtLeg.Columns.Add("Case_Number"); dtLeg.Columns.Add("Departure_Detail_Id"); dtLeg.Columns.Add("Departure_Id"); dtLeg.Columns.Add("Cabin_Class"); dtLeg.Columns.Add("Leg_Segment_type"); dtLeg.Columns.Add("Departure_Detail_Capture_Id");
                dtLeg.Columns.Add("DDC_Compartment_Id"); dtLeg.Columns.Add("Compartment_Structure_Id"); dtLeg.Columns.Add("Compartment_Name"); dtLeg.Columns.Add("Compartment_id"); dtLeg.Columns.Add("DDC_Class_Id"); dtLeg.Columns.Add("Class_Structure_Id");
                dtLeg.Columns.Add("Class_Name"); dtLeg.Columns.Add("Class_id"); dtLeg.Columns.Add("DDC_Subclass_Id"); dtLeg.Columns.Add("Subclass_Structure_Id"); dtLeg.Columns.Add("Subclass_Name"); dtLeg.Columns.Add("Subclass_id");
                dtLeg.Columns.Add("Adjusted_Authorizations"); dtLeg.Columns.Add("Adjusted_Availability"); dtLeg.Columns.Add("Adjusted_Bid_Price"); dtLeg.Columns.Add("Adjusted_Boardings"); dtLeg.Columns.Add("Adjusted_Bookings"); dtLeg.Columns.Add("Adjusted_Load_Factor");
                dtLeg.Columns.Add("Adjusted_Physical_Load_Factor"); dtLeg.Columns.Add("Adjusted_Revenue_Gain"); dtLeg.Columns.Add("Adjusted_SCI"); dtLeg.Columns.Add("Authorized_Capacity"); dtLeg.Columns.Add("Average_Fare_Value"); dtLeg.Columns.Add("Booked_Costs");
                dtLeg.Columns.Add("Booked_Revenue"); dtLeg.Columns.Add("Class_Usage_Ratio"); dtLeg.Columns.Add("Confidence"); dtLeg.Columns.Add("Current_Authorizations"); dtLeg.Columns.Add("Current_Availability"); dtLeg.Columns.Add("Current_Bookings");
                dtLeg.Columns.Add("Current_SCI"); dtLeg.Columns.Add("Go_Shows"); dtLeg.Columns.Add("Group_Bookings"); dtLeg.Columns.Add("Group_No_Show_Percentage"); dtLeg.Columns.Add("Limit_Compartment"); dtLeg.Columns.Add("Max_Constraint_Type");
                dtLeg.Columns.Add("Maximum_Authorizations_Constraint"); dtLeg.Columns.Add("Maximum_Leg_Authorization"); dtLeg.Columns.Add("Min_Constraint_Type"); dtLeg.Columns.Add("Minimum_Authorizations_Constraint"); dtLeg.Columns.Add("No_Show_Percentage"); dtLeg.Columns.Add("No_Shows");
                dtLeg.Columns.Add("Overbooking_Amount"); dtLeg.Columns.Add("Overbooking_Cost"); dtLeg.Columns.Add("Physical_Capacity"); dtLeg.Columns.Add("Projected_Authorized_Load_Factor"); dtLeg.Columns.Add("Projected_Boardings"); dtLeg.Columns.Add("Projected_Bookings");
                dtLeg.Columns.Add("Projected_Physical_Load_Factor"); dtLeg.Columns.Add("Projected_Revenue"); dtLeg.Columns.Add("Recommended_Authorized_Load_Factor"); dtLeg.Columns.Add("Recommended_Authorizations"); dtLeg.Columns.Add("Recommended_Availability"); dtLeg.Columns.Add("Recommended_Bid_Price");
                dtLeg.Columns.Add("Recommended_Boardings"); dtLeg.Columns.Add("Recommended_Bookings"); dtLeg.Columns.Add("Recommended_Physical_Load_Factor"); dtLeg.Columns.Add("Recommended_Revenue"); dtLeg.Columns.Add("Recommended_Revenue_Gain"); dtLeg.Columns.Add("Recommended_SCI");
                dtLeg.Columns.Add("Remaining_Cancellations"); dtLeg.Columns.Add("Original_Demand"); dtLeg.Columns.Add("Remaining_Demand"); dtLeg.Columns.Add("Revenue_Target"); dtLeg.Columns.Add("Standard_Deviation"); dtLeg.Columns.Add("Theoretical_Capacity");
                dtLeg.Columns.Add("Total_Booked_Costs"); dtLeg.Columns.Add("Total_Booked_Revenue"); dtLeg.Columns.Add("Total_Group_Bookings"); dtLeg.Columns.Add("Total_Projected_Demand"); dtLeg.Columns.Add("Total_Waitlist"); dtLeg.Columns.Add("Unit_Target");
                dtLeg.Columns.Add("Upsell_Adjusted_Demand"); dtLeg.Columns.Add("Upsell_Authorizations"); dtLeg.Columns.Add("Upsell_Availability"); dtLeg.Columns.Add("Upsell_Boardings"); dtLeg.Columns.Add("Upsell_Bookings"); dtLeg.Columns.Add("Upsell_Probability");
                dtLeg.Columns.Add("Upsell_Revenue"); dtLeg.Columns.Add("Upsell_Revenue_Gain"); dtLeg.Columns.Add("Upsell_SCI"); dtLeg.Columns.Add("Upsell_Remaining_Demand"); dtLeg.Columns.Add("Usage_Ratio"); dtLeg.Columns.Add("Waitlist");
                dtLeg.Columns.Add("Limit_Class"); dtLeg.Columns.Add("rec_limit"); dtLeg.Columns.Add("curr_limit"); dtLeg.Columns.Add("App_Capacity"); dtLeg.Columns.Add("Rec_Capacity"); dtLeg.Columns.Add("Is_Summary");
                dtLeg.Columns.Add("Current_Resource_Limit1"); dtLeg.Columns.Add("Recommand_Resource_Limit1"); dtLeg.Columns.Add("Upsell_Resource_limit1"); dtLeg.Columns.Add("Adjusted_Resource_limit1"); dtLeg.Columns.Add("Current_Resource_Limit2"); dtLeg.Columns.Add("Recommand_Resource_Limit2");
                dtLeg.Columns.Add("Upsell_Resource_limit2"); dtLeg.Columns.Add("Adjusted_Resource_limit2"); dtLeg.Columns.Add("Include_In_Total_Flag"); dtLeg.Columns.Add("Keep_Curr_Auth"); dtLeg.Columns.Add("Discrete_Recomended_Authorization"); dtLeg.Columns.Add("Discrete_Upsell_Authorization");
                dtLeg.Columns.Add("Nesting_Type"); dtLeg.Columns.Add("BookingPickup_DaysPrior_i"); dtLeg.Columns.Add("BookingPickup_Per_DaysPrior_i"); dtLeg.Columns.Add("BookingPickup_DaysPrior_j"); dtLeg.Columns.Add("BookingPickup_Per_DaysPrior_j"); dtLeg.Columns.Add("BookingPickup_DaysPrior_k");
                dtLeg.Columns.Add("BookingPickup_Per_DaysPrior_k"); dtLeg.Columns.Add("SDL_Booked"); dtLeg.Columns.Add("SDL_Booked_Percentage"); dtLeg.Columns.Add("Travelled_Bookings"); dtLeg.Columns.Add("Travelled_Bookings_Percentage"); dtLeg.Columns.Add("PE_Authorization");
                dtLeg.Columns.Add("PE_Availability"); dtLeg.Columns.Add("PE_Booking"); dtLeg.Columns.Add("PE_Boarding"); dtLeg.Columns.Add("PE_Revenue"); dtLeg.Columns.Add("PE_Revenue_Gain"); dtLeg.Columns.Add("Marginal_Fare");
                dtLeg.Columns.Add("Marginal_Demand"); dtLeg.Columns.Add("Price_Elasticity"); dtLeg.Columns.Add("PE_Group_ID"); dtLeg.Columns.Add("PE_Demand"); dtLeg.Columns.Add("Max_Alloc_Hus"); dtLeg.Columns.Add("PAX_Buffer_Hus");
                dtLeg.Columns.Add("PAX_Buffer_Hus_Type"); dtLeg.Columns.Add("Max_Alloc_Berths"); dtLeg.Columns.Add("PAX_Buffer_Berths"); dtLeg.Columns.Add("PAX_Buffer_Berths_Type"); dtLeg.Columns.Add("IS_Sent"); dtLeg.Columns.Add("StepType");
                dtLeg.Columns.Add("Adjusted_Authorizations_Calc"); dtLeg.Columns.Add("SDL_BP_DP_i"); dtLeg.Columns.Add("SDL_BP_DP_j"); dtLeg.Columns.Add("SDL_BP_DP_k"); dtLeg.Columns.Add("current_revenue"); dtLeg.Columns.Add("revenue_SDL");
                dtLeg.Columns.Add("Diff_Bkd_LY"); dtLeg.Columns.Add("Diff_Bkd_SDL"); dtLeg.Columns.Add("Diff_Revenue"); dtLeg.Columns.Add("Max_Booked");

                DataRow drLeg = dtLeg.NewRow();
                drLeg["Case_Number"] = cNo; drLeg["Departure_Detail_Id"] = dDetID; drLeg["Departure_Id"] = dID; drLeg["Cabin_Class"] = cCls; drLeg["Leg_Segment_type"] = "Leg"; drLeg["Departure_Detail_Capture_Id"] = "43824017"; drLeg["DDC_Compartment_Id"] = System.DBNull.Value;
                drLeg["Compartment_Structure_Id"] = System.DBNull.Value; drLeg["Compartment_Name"] = System.DBNull.Value;


                dtLeg.Rows.Add(drLeg);


                dtComp.Columns.Add("Case_Number"); dtComp.Columns.Add("Departure_Detail_Id"); dtComp.Columns.Add("Departure_Id"); dtComp.Columns.Add("Cabin_Class"); dtComp.Columns.Add("Leg_Segment_type"); dtComp.Columns.Add("Departure_Detail_Capture_Id");
                dtComp.Columns.Add("DDC_Compartment_Id"); dtComp.Columns.Add("Compartment_Structure_Id"); dtComp.Columns.Add("Compartment_Name"); dtComp.Columns.Add("Compartment_id"); dtComp.Columns.Add("DDC_Class_Id"); dtComp.Columns.Add("Class_Structure_Id");
                dtComp.Columns.Add("Class_Name"); dtComp.Columns.Add("Class_id"); dtComp.Columns.Add("DDC_Subclass_Id"); dtComp.Columns.Add("Subclass_Structure_Id"); dtComp.Columns.Add("Subclass_Name"); dtComp.Columns.Add("Subclass_id");
                dtComp.Columns.Add("Adjusted_Authorizations"); dtComp.Columns.Add("Adjusted_Availability"); dtComp.Columns.Add("Adjusted_Bid_Price"); dtComp.Columns.Add("Adjusted_Boardings"); dtComp.Columns.Add("Adjusted_Bookings"); dtComp.Columns.Add("Adjusted_Load_Factor");
                dtComp.Columns.Add("Adjusted_Physical_Load_Factor"); dtComp.Columns.Add("Adjusted_Revenue"); dtComp.Columns.Add("Adjusted_Revenue_Gain"); dtComp.Columns.Add("Adjusted_SCI"); dtComp.Columns.Add("Authorized_Capacity"); dtComp.Columns.Add("Average_Fare_Value"); dtComp.Columns.Add("Booked_Costs");
                dtComp.Columns.Add("Booked_Revenue"); dtComp.Columns.Add("Class_Usage_Ratio"); dtComp.Columns.Add("Confidence"); dtComp.Columns.Add("Current_Authorizations"); dtComp.Columns.Add("Current_Availability"); dtComp.Columns.Add("Current_Bookings");
                dtComp.Columns.Add("Current_SCI"); dtComp.Columns.Add("Go_Shows"); dtComp.Columns.Add("Group_Bookings"); dtComp.Columns.Add("Group_No_Show_Percentage"); dtComp.Columns.Add("Limit_Compartment"); dtComp.Columns.Add("Max_Constraint_Type");
                dtComp.Columns.Add("Maximum_Authorizations_Constraint"); dtComp.Columns.Add("Maximum_Leg_Authorization"); dtComp.Columns.Add("Min_Constraint_Type"); dtComp.Columns.Add("Minimum_Authorizations_Constraint"); dtComp.Columns.Add("No_Show_Percentage"); dtComp.Columns.Add("No_Shows");
                dtComp.Columns.Add("Overbooking_Amount"); dtComp.Columns.Add("Overbooking_Cost"); dtComp.Columns.Add("Physical_Capacity"); dtComp.Columns.Add("Projected_Authorized_Load_Factor"); dtComp.Columns.Add("Projected_Boardings"); dtComp.Columns.Add("Projected_Bookings");
                dtComp.Columns.Add("Projected_Physical_Load_Factor"); dtComp.Columns.Add("Projected_Revenue"); dtComp.Columns.Add("Recommended_Authorized_Load_Factor"); dtComp.Columns.Add("Recommended_Authorizations"); dtComp.Columns.Add("Recommended_Availability"); dtComp.Columns.Add("Recommended_Bid_Price");
                dtComp.Columns.Add("Recommended_Boardings"); dtComp.Columns.Add("Recommended_Bookings"); dtComp.Columns.Add("Recommended_Physical_Load_Factor"); dtComp.Columns.Add("Recommended_Revenue"); dtComp.Columns.Add("Recommended_Revenue_Gain"); dtComp.Columns.Add("Recommended_SCI");
                dtComp.Columns.Add("Remaining_Cancellations"); dtComp.Columns.Add("Original_Demand"); dtComp.Columns.Add("Remaining_Demand"); dtComp.Columns.Add("Revenue_Target"); dtComp.Columns.Add("Standard_Deviation"); dtComp.Columns.Add("Theoretical_Capacity");
                dtComp.Columns.Add("Total_Booked_Costs"); dtComp.Columns.Add("Total_Booked_Revenue"); dtComp.Columns.Add("Total_Group_Bookings"); dtComp.Columns.Add("Total_Projected_Demand"); dtComp.Columns.Add("Total_Waitlist"); dtComp.Columns.Add("Unit_Target");
                dtComp.Columns.Add("Upsell_Adjusted_Demand"); dtComp.Columns.Add("Upsell_Authorizations"); dtComp.Columns.Add("Upsell_Availability"); dtComp.Columns.Add("Upsell_Boardings"); dtComp.Columns.Add("Upsell_Bookings"); dtComp.Columns.Add("Upsell_Probability");
                dtComp.Columns.Add("Upsell_Revenue"); dtComp.Columns.Add("Upsell_Revenue_Gain"); dtComp.Columns.Add("Upsell_SCI"); dtComp.Columns.Add("Upsell_Remaining_Demand"); dtComp.Columns.Add("Usage_Ratio"); dtComp.Columns.Add("Waitlist");
                dtComp.Columns.Add("Limit_Class"); dtComp.Columns.Add("rec_limit"); dtComp.Columns.Add("curr_limit"); dtComp.Columns.Add("App_Capacity"); dtComp.Columns.Add("Rec_Capacity"); dtComp.Columns.Add("Is_Summary");
                dtComp.Columns.Add("Current_Resource_Limit1"); dtComp.Columns.Add("Recommand_Resource_Limit1"); dtComp.Columns.Add("Upsell_Resource_limit1"); dtComp.Columns.Add("Adjusted_Resource_limit1"); dtComp.Columns.Add("Current_Resource_Limit2"); dtComp.Columns.Add("Recommand_Resource_Limit2");
                dtComp.Columns.Add("Upsell_Resource_limit2"); dtComp.Columns.Add("Adjusted_Resource_limit2"); dtComp.Columns.Add("Include_In_Total_Flag"); dtComp.Columns.Add("Keep_Curr_Auth"); dtComp.Columns.Add("Discrete_Recomended_Authorization"); dtComp.Columns.Add("Discrete_Upsell_Authorization");
                dtComp.Columns.Add("Nesting_Type"); dtComp.Columns.Add("BookingPickup_DaysPrior_i"); dtComp.Columns.Add("BookingPickup_Per_DaysPrior_i"); dtComp.Columns.Add("BookingPickup_DaysPrior_j"); dtComp.Columns.Add("BookingPickup_Per_DaysPrior_j"); dtComp.Columns.Add("BookingPickup_DaysPrior_k");
                dtComp.Columns.Add("BookingPickup_Per_DaysPrior_k"); dtComp.Columns.Add("SDL_Booked"); dtComp.Columns.Add("SDL_Booked_Percentage"); dtComp.Columns.Add("Travelled_Bookings"); dtComp.Columns.Add("Travelled_Bookings_Percentage"); dtComp.Columns.Add("PE_Authorization");
                dtComp.Columns.Add("PE_Availability"); dtComp.Columns.Add("PE_Booking"); dtComp.Columns.Add("PE_Boarding"); dtComp.Columns.Add("PE_Revenue"); dtComp.Columns.Add("PE_Revenue_Gain"); dtComp.Columns.Add("Marginal_Fare");
                dtComp.Columns.Add("Marginal_Demand"); dtComp.Columns.Add("Price_Elasticity"); dtComp.Columns.Add("PE_Group_ID"); dtComp.Columns.Add("PE_Demand"); dtComp.Columns.Add("Max_Alloc_Hus"); dtComp.Columns.Add("PAX_Buffer_Hus");
                dtComp.Columns.Add("PAX_Buffer_Hus_Type"); dtComp.Columns.Add("Max_Alloc_Berths"); dtComp.Columns.Add("PAX_Buffer_Berths"); dtComp.Columns.Add("PAX_Buffer_Berths_Type"); dtComp.Columns.Add("IS_Sent"); dtComp.Columns.Add("StepType");
                dtComp.Columns.Add("Adjusted_Authorizations_Calc"); dtComp.Columns.Add("SDL_BP_DP_i"); dtComp.Columns.Add("SDL_BP_DP_j"); dtComp.Columns.Add("SDL_BP_DP_k"); dtComp.Columns.Add("current_revenue"); dtComp.Columns.Add("revenue_SDL");
                dtComp.Columns.Add("Diff_Bkd_LY"); dtComp.Columns.Add("Diff_Bkd_SDL"); dtComp.Columns.Add("Diff_Revenue"); dtComp.Columns.Add("Max_Booked"); dtComp.Columns.Add("COMPARTMENT_STRUCTURE_COMPARTMENT_ORDER");

               

                DataRow drComp = dtComp.NewRow();
                drComp["Case_Number"] = cNo; drComp["Departure_Detail_Id"] = dDetID; drComp["Departure_Id"] = dID; drComp["Cabin_Class"] = cCls; drComp["Leg_Segment_type"] = "Leg"; drComp["Departure_Detail_Capture_Id"] = "43824017"; drComp["DDC_Compartment_Id"] = "60513359";
                drComp["Compartment_Structure_Id"] = 4; drComp["Compartment_Name"] = "J"; drComp["Compartment_id"] = 1; drComp["Adjusted_Authorizations"] = 8; drComp["Adjusted_Availability"] = 7; drComp["Adjusted_Boardings"] = 1; drComp["Adjusted_Bookings"] = 1;
                drComp["Adjusted_Load_Factor"] = 0; drComp["Adjusted_Revenue"] = 349; drComp["Current_Authorizations"] = 8; drComp["Current_Availability"] = 7;

                dtComp.Rows.Add(drComp);

                DataRow drComp1 = dtComp.NewRow();
                drComp1["Case_Number"] = cNo; drComp1["Departure_Detail_Id"] = dDetID; drComp1["Departure_Id"] = dID; drComp1["Cabin_Class"] = cCls; drComp1["Leg_Segment_type"] = "Leg"; drComp1["Departure_Detail_Capture_Id"] = "43824017"; drComp1["DDC_Compartment_Id"] = "60513360";
                drComp1["Compartment_Structure_Id"] = 5; drComp1["Compartment_Name"] = "Y"; drComp1["Compartment_id"] = 2; drComp1["Adjusted_Authorizations"] = 114; drComp1["Adjusted_Availability"] = 97; drComp1["Adjusted_Boardings"] = 39; drComp1["Adjusted_Bookings"] = 39;
                drComp1["Adjusted_Load_Factor"] = 0; drComp1["Adjusted_Revenue"] = 350; drComp1["Current_Authorizations"] = 10; drComp1["Current_Availability"] = 10;

                dtComp.Rows.Add(drComp1);

                dtClass.Columns.Add("Case_Number"); dtClass.Columns.Add("Departure_Detail_Id"); dtClass.Columns.Add("Departure_Id"); dtClass.Columns.Add("Cabin_Class"); dtClass.Columns.Add("Leg_Segment_type"); dtClass.Columns.Add("Departure_Detail_Capture_Id");
                dtClass.Columns.Add("DDC_Compartment_Id"); dtClass.Columns.Add("Compartment_Structure_Id"); dtClass.Columns.Add("Compartment_Name"); dtClass.Columns.Add("Compartment_id"); dtClass.Columns.Add("DDC_Class_Id"); dtClass.Columns.Add("Class_Structure_Id");
                dtClass.Columns.Add("Class_Name"); dtClass.Columns.Add("Class_id"); dtClass.Columns.Add("DDC_Subclass_Id"); dtClass.Columns.Add("Subclass_Structure_Id"); dtClass.Columns.Add("Subclass_Name"); dtClass.Columns.Add("Subclass_id");
                dtClass.Columns.Add("Adjusted_Authorizations"); dtClass.Columns.Add("Adjusted_Availability"); dtClass.Columns.Add("Adjusted_Bid_Price"); dtClass.Columns.Add("Adjusted_Boardings"); dtClass.Columns.Add("Adjusted_Bookings"); dtClass.Columns.Add("Adjusted_Load_Factor");
                dtClass.Columns.Add("Adjusted_Physical_Load_Factor"); dtClass.Columns.Add("Adjusted_Revenue"); dtClass.Columns.Add("Adjusted_Revenue_Gain"); dtClass.Columns.Add("Adjusted_SCI"); dtClass.Columns.Add("Authorized_Capacity"); dtClass.Columns.Add("Average_Fare_Value"); dtClass.Columns.Add("Booked_Costs");
                dtClass.Columns.Add("Booked_Revenue"); dtClass.Columns.Add("Class_Usage_Ratio"); dtClass.Columns.Add("Confidence"); dtClass.Columns.Add("Current_Authorizations"); dtClass.Columns.Add("Current_Availability"); dtClass.Columns.Add("Current_Bookings");
                dtClass.Columns.Add("Current_SCI"); dtClass.Columns.Add("Go_Shows"); dtClass.Columns.Add("Group_Bookings"); dtClass.Columns.Add("Group_No_Show_Percentage"); dtClass.Columns.Add("Limit_Compartment"); dtClass.Columns.Add("Max_Constraint_Type");
                dtClass.Columns.Add("Maximum_Authorizations_Constraint"); dtClass.Columns.Add("Maximum_Leg_Authorization"); dtClass.Columns.Add("Min_Constraint_Type"); dtClass.Columns.Add("Minimum_Authorizations_Constraint"); dtClass.Columns.Add("No_Show_Percentage"); dtClass.Columns.Add("No_Shows");
                dtClass.Columns.Add("Overbooking_Amount"); dtClass.Columns.Add("Overbooking_Cost"); dtClass.Columns.Add("Physical_Capacity"); dtClass.Columns.Add("Projected_Authorized_Load_Factor"); dtClass.Columns.Add("Projected_Boardings"); dtClass.Columns.Add("Projected_Bookings");
                dtClass.Columns.Add("Projected_Physical_Load_Factor"); dtClass.Columns.Add("Projected_Revenue"); dtClass.Columns.Add("Recommended_Authorized_Load_Factor"); dtClass.Columns.Add("Recommended_Authorizations"); dtClass.Columns.Add("Recommended_Availability"); dtClass.Columns.Add("Recommended_Bid_Price");
                dtClass.Columns.Add("Recommended_Boardings"); dtClass.Columns.Add("Recommended_Bookings"); dtClass.Columns.Add("Recommended_Physical_Load_Factor"); dtClass.Columns.Add("Recommended_Revenue"); dtClass.Columns.Add("Recommended_Revenue_Gain"); dtClass.Columns.Add("Recommended_SCI");
                dtClass.Columns.Add("Remaining_Cancellations"); dtClass.Columns.Add("Original_Demand"); dtClass.Columns.Add("Remaining_Demand"); dtClass.Columns.Add("Revenue_Target"); dtClass.Columns.Add("Standard_Deviation"); dtClass.Columns.Add("Theoretical_Capacity");
                dtClass.Columns.Add("Total_Booked_Costs"); dtClass.Columns.Add("Total_Booked_Revenue"); dtClass.Columns.Add("Total_Group_Bookings"); dtClass.Columns.Add("Total_Projected_Demand"); dtClass.Columns.Add("Total_Waitlist"); dtClass.Columns.Add("Unit_Target");
                dtClass.Columns.Add("Upsell_Adjusted_Demand"); dtClass.Columns.Add("Upsell_Authorizations"); dtClass.Columns.Add("Upsell_Availability"); dtClass.Columns.Add("Upsell_Boardings"); dtClass.Columns.Add("Upsell_Bookings"); dtClass.Columns.Add("Upsell_Probability");
                dtClass.Columns.Add("Upsell_Revenue"); dtClass.Columns.Add("Upsell_Revenue_Gain"); dtClass.Columns.Add("Upsell_SCI"); dtClass.Columns.Add("Upsell_Remaining_Demand"); dtClass.Columns.Add("Usage_Ratio"); dtClass.Columns.Add("Waitlist");
                dtClass.Columns.Add("Limit_Class"); dtClass.Columns.Add("rec_limit"); dtClass.Columns.Add("curr_limit"); dtClass.Columns.Add("App_Capacity"); dtClass.Columns.Add("Rec_Capacity"); dtClass.Columns.Add("Is_Summary");
                dtClass.Columns.Add("Current_Resource_Limit1"); dtClass.Columns.Add("Recommand_Resource_Limit1"); dtClass.Columns.Add("Upsell_Resource_limit1"); dtClass.Columns.Add("Adjusted_Resource_limit1"); dtClass.Columns.Add("Current_Resource_Limit2"); dtClass.Columns.Add("Recommand_Resource_Limit2");
                dtClass.Columns.Add("Upsell_Resource_limit2"); dtClass.Columns.Add("Adjusted_Resource_limit2"); dtClass.Columns.Add("Include_In_Total_Flag"); dtClass.Columns.Add("Keep_Curr_Auth"); dtClass.Columns.Add("Discrete_Recomended_Authorization"); dtClass.Columns.Add("Discrete_Upsell_Authorization");
                dtClass.Columns.Add("Nesting_Type"); dtClass.Columns.Add("BookingPickup_DaysPrior_i"); dtClass.Columns.Add("BookingPickup_Per_DaysPrior_i"); dtClass.Columns.Add("BookingPickup_DaysPrior_j"); dtClass.Columns.Add("BookingPickup_Per_DaysPrior_j"); dtClass.Columns.Add("BookingPickup_DaysPrior_k");
                dtClass.Columns.Add("BookingPickup_Per_DaysPrior_k"); dtClass.Columns.Add("SDL_Booked"); dtClass.Columns.Add("SDL_Booked_Percentage"); dtClass.Columns.Add("Travelled_Bookings"); dtClass.Columns.Add("Travelled_Bookings_Percentage"); dtClass.Columns.Add("PE_Authorization");
                dtClass.Columns.Add("PE_Availability"); dtClass.Columns.Add("PE_Booking"); dtClass.Columns.Add("PE_Boarding"); dtClass.Columns.Add("PE_Revenue"); dtClass.Columns.Add("PE_Revenue_Gain"); dtClass.Columns.Add("Marginal_Fare");
                dtClass.Columns.Add("Marginal_Demand"); dtClass.Columns.Add("Price_Elasticity"); dtClass.Columns.Add("PE_Group_ID"); dtClass.Columns.Add("PE_Demand"); dtClass.Columns.Add("Max_Alloc_Hus"); dtClass.Columns.Add("PAX_Buffer_Hus");
                dtClass.Columns.Add("PAX_Buffer_Hus_Type"); dtClass.Columns.Add("Max_Alloc_Berths"); dtClass.Columns.Add("PAX_Buffer_Berths"); dtClass.Columns.Add("PAX_Buffer_Berths_Type"); dtClass.Columns.Add("IS_Sent"); dtClass.Columns.Add("StepType");
                dtClass.Columns.Add("Adjusted_Authorizations_Calc"); dtClass.Columns.Add("SDL_BP_DP_i"); dtClass.Columns.Add("SDL_BP_DP_j"); dtClass.Columns.Add("SDL_BP_DP_k"); dtClass.Columns.Add("current_revenue"); dtClass.Columns.Add("revenue_SDL");
                dtClass.Columns.Add("Diff_Bkd_LY"); dtClass.Columns.Add("Diff_Bkd_SDL"); dtClass.Columns.Add("Diff_Revenue"); dtClass.Columns.Add("Max_Booked"); dtClass.Columns.Add("COMPARTMENT_STRUCTURE_COMPARTMENT_ORDER");


                DataRow drClass = dtClass.NewRow();
                drClass["Case_Number"] = cNo; drClass["Departure_Detail_Id"] = dDetID; drClass["Departure_Id"] = dID; drClass["Cabin_Class"] = cCls; drClass["Leg_Segment_type"] = "Leg"; drClass["Departure_Detail_Capture_Id"] = "43824017"; drClass["DDC_Compartment_Id"] = "60513360";
                drClass["Compartment_Structure_Id"] = 5; drClass["Compartment_Name"] = "Y"; drClass["Compartment_id"] = 2; drClass["DDC_Class_Id"] = "6022025"; drClass["Class_Structure_Id"] = 4;
                drClass["Class_Name"] = "Y"; drClass["Class_id"] = 4; drClass["Adjusted_Authorizations"] = 8; drClass["Adjusted_Availability"] = 7; drClass["Adjusted_Boardings"] = 1; drClass["Adjusted_Bookings"] = 1;
                drClass["Adjusted_Load_Factor"] = 0; drClass["Adjusted_Revenue"] = 349; drClass["Current_Authorizations"] = 8; drClass["Current_Availability"] = 7;

                dtClass.Rows.Add(drClass);

                DataRow drClass1 = dtClass.NewRow();
                drClass1["Case_Number"] = cNo; drClass1["Departure_Detail_Id"] = dDetID; drClass1["Departure_Id"] = dID; drClass1["Cabin_Class"] = cCls; drClass1["Leg_Segment_type"] = "Leg"; drClass1["Departure_Detail_Capture_Id"] = "43824017"; drClass1["DDC_Compartment_Id"] = "60513359";
                drClass1["Compartment_Structure_Id"] = 4; drClass1["Compartment_Name"] = "J"; drClass1["Compartment_id"] = 1; drClass1["DDC_Class_Id"] = "6022020"; drClass1["Class_Structure_Id"] = 1;
                drClass1["Class_Name"] = "J"; drClass1["Class_id"] = 1; drClass1["Adjusted_Authorizations"] = 8; drClass1["Adjusted_Availability"] = 7; drClass1["Adjusted_Boardings"] = 1; drClass1["Adjusted_Bookings"] = 1;
                drClass1["Adjusted_Load_Factor"] = 0; drClass1["Adjusted_Revenue"] = 349; drClass1["Current_Authorizations"] = 8; drClass1["Current_Availability"] = 7;

                dtClass.Rows.Add(drClass1);

                DataRow drClass2 = dtClass.NewRow();
                drClass2["Case_Number"] = cNo; drClass2["Departure_Detail_Id"] = dDetID; drClass2["Departure_Id"] = dID; drClass2["Cabin_Class"] = cCls; drClass2["Leg_Segment_type"] = "Leg"; drClass2["Departure_Detail_Capture_Id"] = "43824017"; drClass2["DDC_Compartment_Id"] = "60513359";
                drClass2["Compartment_Structure_Id"] = 4; drClass2["Compartment_Name"] = "J"; drClass2["Compartment_id"] = 1; drClass2["DDC_Class_Id"] = "6022020"; drClass2["Class_Structure_Id"] = 17;
                drClass2["Class_Name"] = "C"; drClass2["Class_id"] = 17; drClass2["Adjusted_Authorizations"] = 8; drClass2["Adjusted_Availability"] = 7; drClass2["Adjusted_Boardings"] = 1; drClass2["Adjusted_Bookings"] = 1;
                drClass2["Adjusted_Load_Factor"] = 0; drClass2["Adjusted_Revenue"] = 349; drClass2["Current_Authorizations"] = 8; drClass2["Current_Availability"] = 7;

                dtClass.Rows.Add(drClass2);


                BindGrid();
            }
            catch(Exception)
            {
                throw;
            }
        }

        private void BindGrid()
        {
            try
            {

                radGridWorksheet.MasterTemplate.Templates.Clear();
                radGridWorksheet.Relations.Clear();
                radGridWorksheet.DataSource = dtLeg;
                radGridWorksheet.AllowAddNewRow = false;
                radGridWorksheet.AllowDeleteRow = false;
                radGridWorksheet.AllowEditRow = true;
                radGridWorksheet.AutoGenerateColumns = true;
                radGridWorksheet.BestFitColumns();
                radGridWorksheet.GridViewElement.DrawBorder = false;
                radGridWorksheet.GridViewElement.GroupPanelElement.DrawBorder = false;

                GridViewTemplate childDetailCaptureDate = new GridViewTemplate();
                childDetailCaptureDate.Caption = "Leg";
                childDetailCaptureDate.Columns.Clear();
                childDetailCaptureDate.AllowAddNewRow = false;
                childDetailCaptureDate.AllowDeleteRow = false;
                childDetailCaptureDate.AllowEditRow = true;
                childDetailCaptureDate.AutoGenerateColumns = true;
                childDetailCaptureDate.BestFitColumns();
                childDetailCaptureDate.DataSource = dtComp;
                radGridWorksheet.MasterTemplate.Templates.Add(childDetailCaptureDate);
                GridViewRelation relationCaptureDate = new GridViewRelation(radGridWorksheet.MasterTemplate);
                relationCaptureDate.ChildTemplate = childDetailCaptureDate;
                relationCaptureDate.RelationName = "Capture";
                relationCaptureDate.ParentColumnNames.Add("Departure_Detail_Capture_Id");
                relationCaptureDate.ChildColumnNames.Add("Departure_Detail_Capture_Id");
                radGridWorksheet.Relations.Add(relationCaptureDate);

                GridViewTemplate childDetailCaptureDateComp = new GridViewTemplate();
                childDetailCaptureDateComp.Caption = "Comp";
                childDetailCaptureDateComp.Columns.Clear();
                childDetailCaptureDateComp.AllowAddNewRow = false;
                childDetailCaptureDateComp.AllowDeleteRow = false;
                childDetailCaptureDateComp.AllowEditRow = true;
                childDetailCaptureDateComp.AutoGenerateColumns = true;
                childDetailCaptureDateComp.BestFitColumns();
                childDetailCaptureDateComp.DataSource = dtClass;
                childDetailCaptureDate.Templates.Add(childDetailCaptureDateComp);
                GridViewRelation relationCaptureDateCompatment = new GridViewRelation(childDetailCaptureDate);
                relationCaptureDateCompatment.ChildTemplate = childDetailCaptureDateComp;
                relationCaptureDateCompatment.RelationName = "Compartment";
                relationCaptureDateCompatment.ParentColumnNames.Add("DDC_Compartment_Id");
                relationCaptureDateCompatment.ChildColumnNames.Add("DDC_Compartment_Id");
                radGridWorksheet.Relations.Add(relationCaptureDateCompatment); 

                radGridWorksheet.MasterTemplate.ExpandAll();

            }
            catch(Exception e0)
            { 
                MessageBox.Show(String.Format("{0}\n{1}", e0.Message, e0.StackTrace));
                throw;

            }
        }

        private void radButton1_Click(object sender, EventArgs e)
        {
            try
            {
                intClickCount = intClickCount + 1;
                //Thread.Sleep(1000);
                radButton1.Text = "Next Click " + intClickCount;
                BindTables();
            }
            catch(Exception e0)
            { 
                MessageBox.Show(String.Format("{0}\n{1}", e0.Message, e0.StackTrace));
                throw;

            }

        }
    }

Setting the FormatString of a column in the child template does not seem to take effect.

Workaround: handle the SortChanging and FilterChanging events and clear the search criteria from the search row. The stored search string can later be restored in the SortChanged and FilterChanged events

string filter = string.Empty;
private void RadGrid_FilterChanged(object sender, GridViewCollectionChangedEventArgs e)
{
    this.radGrid.MasterView.TableSearchRow.Search(filter);
}

private void RadGrid_SortChanged(object sender, GridViewCollectionChangedEventArgs e)
{
    this.radGrid.MasterView.TableSearchRow.Search(filter);
}

private void RadGrid_SortChanging(object sender, GridViewCollectionChangingEventArgs e)
{
    this.filter = this.radGrid.MasterView.TableSearchRow.SearchCriteria;
    this.radGrid.MasterView.TableSearchRow.Search(null);
}

private void RadGrid_FilterChanging(object sender, GridViewCollectionChangingEventArgs e)
{
    this.filter = this.radGrid.MasterView.TableSearchRow.SearchCriteria;
    this.radGrid.MasterView.TableSearchRow.Search(null);
}

To reproduce, add menu item in the filtering context menu and upon click, set it as checked. Note that the filtering cell text should also take the custom menu item text or there should be an approach to set it.
Workaround: uncomment the commented code below.

protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);

            AddGrid();

            DataTable dt = new DataTable();
            dt.Columns.Add("Id", typeof(int));
            dt.Columns.Add("Name", typeof(string));

            for (int i = 0; i < 5; i++)
            {
                dt.Rows.Add(i, "Item" + i);
            }
            dt.Rows.Add(5, null);
            dt.Rows.Add(6, "");

            radGridView1.ShowHeaderCellButtons = true;
            this.radGridView1.DataSource = dt;
            this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
            this.radGridView1.EnableFiltering = true;
            this.radGridView1.EnableCustomFiltering = true;
            //radGridView1.ContextMenuOpening += radGridView1_ContextMenuOpening;
        }

        private void radGridView1_ContextMenuOpening(object sender, Telerik.WinControls.UI.ContextMenuOpeningEventArgs e)
        {
            GridFilterCellElement filterCell = e.ContextMenuProvider as GridFilterCellElement;
            GridViewDataColumn dataCol = (GridViewDataColumn)filterCell.ColumnInfo;
            if (filterCell != null)
            {
                if (filterCell.ColumnInfo.Name == "Name")
                {
                    RadMenuItem isNullMenuItem = new RadMenuItem();
                    isNullMenuItem.Click += customMenuItem_Click;
                    isNullMenuItem.Text = "My IsNull";
                    isNullMenuItem.Tag = filterCell.ColumnInfo.Name;
                    e.ContextMenu.Items.Add(isNullMenuItem);

                    RadMenuItem isNotNullMenuItem = new RadMenuItem();
                    isNotNullMenuItem.Click += customMenuItem_Click;
                    isNotNullMenuItem.Text = "My IsNotNull";
                    isNotNullMenuItem.Tag = filterCell.ColumnInfo.Name;
                    e.ContextMenu.Items.Add(isNotNullMenuItem);

                    bool isCustomFilter = (dataCol.FilterDescriptor != null &&
                        dataCol.FilterDescriptor is CompositeFilterDescriptor &&
                        dataCol.FilterDescriptor.Expression.Contains("NULL"));

                    foreach (RadMenuItem item in e.ContextMenu.Items)
                    {
                        if (item.Text == "Is null" || item.Text == "Is not null")
                        {
                            item.Visibility = Telerik.WinControls.ElementVisibility.Collapsed;
                        }
                        //if (isCustomFilter)
                        //{
                        //    item.IsChecked = false;
                        //}
                    }
                    //if (isCustomFilter)
                    //{
                    //    if (((CompositeFilterDescriptor)dataCol.FilterDescriptor).LogicalOperator == FilterLogicalOperator.And)
                    //    {
                    //        isNotNullMenuItem.IsChecked = true;
                    //    }
                    //    else
                    //    {
                    //        isNullMenuItem.IsChecked = true;
                    //    }
                    //}
                }
            }
        }

        private void customMenuItem_Click(object sender, EventArgs e)
        {
            RadMenuItem clickedItem = (RadMenuItem)sender;
            string columnName = clickedItem.Tag.ToString();
            radGridView1.Columns[columnName].FilterDescriptor = null;

            FilterOperator filterOperator = clickedItem.Text.Contains("Not") ? FilterOperator.IsNotEqualTo : FilterOperator.IsEqualTo;
            FilterLogicalOperator logicalOperator = clickedItem.Text.Contains("Not") ? FilterLogicalOperator.And : FilterLogicalOperator.Or;

            CompositeFilterDescriptor compositeFilter = new CompositeFilterDescriptor();
            compositeFilter.FilterDescriptors.Add(new FilterDescriptor(columnName, filterOperator, ""));
            compositeFilter.FilterDescriptors.Add(new FilterDescriptor(columnName, filterOperator, null));
            compositeFilter.LogicalOperator = logicalOperator;
            compositeFilter.IsFilterEditor = true;
            this.radGridView1.FilterDescriptors.Add(compositeFilter);

            clickedItem.IsChecked = true;
        }

To reproduce: set Excel-like filter according to the online documentation: https://docs.telerik.com/devtools/winforms/gridview/filtering/setting-filters-programmatically-(composite-descriptors)

            this.radGridView1.DataSource = this.productsBindingSource;
            this.productsTableAdapter.Fill(this.nwindDataSet.Products);

            this.radGridView1.EnableFiltering = true;
            this.radGridView1.ShowHeaderCellButtons = true;
            
            var filterDescriptor = new FilterDescriptor();
            filterDescriptor.PropertyName = "UnitsInStock";
            filterDescriptor.Value = 101;
            filterDescriptor.Operator = FilterOperator.IsEqualTo;

            var filterDescriptor2 = new FilterDescriptor();
            filterDescriptor2.PropertyName = "UnitsInStock";
            filterDescriptor2.Value = 104;
            filterDescriptor2.Operator = FilterOperator.IsEqualTo;
            var cfd = new CompositeFilterDescriptor();
            cfd.LogicalOperator = FilterLogicalOperator.Or;
            cfd.FilterDescriptors.Add(filterDescriptor);
            cfd.FilterDescriptors.Add(filterDescriptor2);
            cfd.IsFilterEditor = true;
            this.radGridView1.FilterDescriptors.Add(cfd);
            this.radGridView1.MasterTemplate.ExcelFilteredColumns.Add(this.radGridView1.Columns["UnitsInStock"]);

You will notice that the filter popup contains all possible values for this column which is OK, but none of the nodes is checked. It is necessary the visible rows to be checked. 

Workaround: check the nodes programmatically when the popup is shown:

        private void radGridView1_FilterPopupInitialized(object sender, Telerik.WinControls.UI.FilterPopupInitializedEventArgs e)
        {
            RadListFilterPopup popup = e.FilterPopup as RadListFilterPopup;
            if (popup!=null)
            {
                popup.Tag = e.Column.Name;
               popup.PopupOpened-=popup_PopupOpened;
                popup.PopupOpened+=popup_PopupOpened;
            }
        }

        private void popup_PopupOpened(object sender, EventArgs args)
        {
           RadListFilterPopup popup = sender as RadListFilterPopup;

           foreach (GridViewRowInfo row in this.radGridView1.ChildRows)
           {
             RadTreeNode node=   popup.MenuTreeElement.TreeView.FindNodes(row.Cells[popup.Tag+""].Value.ToString()).FirstOrDefault();
             if (node!=null)
             {
                 node.Checked = true;
             }
           }
        }

To reproduce: run the attached sample project. Type "res" in the search box very very fast. You will notice that the results with starting with "re" are highlighted. 

Note: it is usually reproducible the first serahc after loading the form. 

How to reproduce: 

public partial class RadForm1 : Telerik.WinControls.UI.RadForm
{
    public RadForm1()
    {
        InitializeComponent();

        this.radGridView1.DataSource = this.GetData();
        this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;

        GridViewDecimalColumn decimalColumn  = this.radGridView1.Columns[0] as GridViewDecimalColumn;
        decimalColumn.DecimalPlaces = 2;
        decimalColumn.FormatString = "{0:N2}";
        decimalColumn.ExcelExportType = DisplayFormatType.Custom;
        decimalColumn.ExcelExportFormatString = "0.000";
    }

    public object GridViewSpreaExport { get; private set; }

    private object GetData()
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Id", typeof(double));
        dt.Columns.Add("Name", typeof(string));
        dt.Columns.Add("Bool", typeof(bool));
        dt.Columns.Add("Date", typeof(DateTime));

        for (int i = 0; i < 100; i++)
        {
            dt.Rows.Add(1.10 + i, "Name " + i, i % 2 == 0, DateTime.Now.AddDays(i));
        }

        return dt;
    }

    private void radButton1_Click(object sender, EventArgs e)
    {
        GridViewSpreadStreamExport spreadExporter = new GridViewSpreadStreamExport(this.radGridView1);
        spreadExporter.FileExportMode = FileExportMode.CreateOrOverrideFile;
        SpreadStreamExportRenderer renderer1 = new SpreadStreamExportRenderer();
        spreadExporter.RunExportAsync(@"..\..\exported-stream.xlsx", renderer1);

    }
}

Workaround: Create a custom SpreadStreamExportRenderer

public class CustomSpreadStreamExportRenderer : SpreadStreamExportRenderer
{
    public override void SetCellValue(DataType dataType, object value)
    {
        switch (dataType)
        {
            case DataType.Number:
                this.SetNumberValue(value);
                break;
            case DataType.DateTime:
                this.SetDateTimeValue(value);
                break;
            case DataType.Boolean:
                this.SetBooleanValue(value);
                break;
            case DataType.Other:
                if (this.SetNumberValue(value))
                {
                    break;
                }
                if (this.SetDateTimeValue(value))
                {
                    break;
                }
                if (this.SetBooleanValue(value))
                {
                    break;
                }

                this.SetStringValue(value);
                break;
            case DataType.String:
            default:
                this.SetStringValue(value);
                break;
        }
    }
}

To reproduce:
private void MasterTemplate_ViewChanged(object sender, DataViewChangedEventArgs args)
{
    if (args.Action == ViewChangedAction.ExpandedChanged)
    {

    }
}
Workaround:
- Use the GroupExpanded event.

To reproduce: please refer to the attached gif file illustrating the steps how to reproduce the problem with the Demo application. 

Workaround:

        private void radGridView1_CellFormatting(object sender, CellFormattingEventArgs e)
        {
            GridDataCellElement cell = e.CellElement as GridDataCellElement;
            if (cell != null)
            {
                if (cell.ContainsErrors)
                {
                    cell.DrawBorder = true;
                    cell.BorderBoxStyle = BorderBoxStyle.FourBorders;

                    cell.BorderBottomColor = cell.BorderTopColor = cell.BorderRightShadowColor = cell.BorderLeftShadowColor = Color.Transparent;
                    cell.BorderBottomShadowColor = cell.BorderTopShadowColor = cell.BorderRightColor = cell.BorderLeftColor = Color.Red;
                    cell.BorderBottomWidth = cell.BorderTopWidth = cell.BorderRightWidth = cell.BorderLeftWidth = 1;

                    cell.BorderDrawMode = BorderDrawModes.HorizontalOverVertical;
                    cell.ZIndex = 2;
                }
                else
                {
                    cell.ResetValue(LightVisualElement.DrawBorderProperty, ValueResetFlags.Local);
                    cell.ResetValue(LightVisualElement.BorderBoxStyleProperty, ValueResetFlags.Local);

                    cell.ResetValue(LightVisualElement.BorderBottomColorProperty, ValueResetFlags.Local);
                    cell.ResetValue(LightVisualElement.BorderBottomShadowColorProperty, ValueResetFlags.Local);
                    cell.ResetValue(LightVisualElement.BorderBottomWidthProperty, ValueResetFlags.Local);

                    cell.ResetValue(LightVisualElement.BorderTopColorProperty, ValueResetFlags.Local);
                    cell.ResetValue(LightVisualElement.BorderTopShadowColorProperty, ValueResetFlags.Local);
                    cell.ResetValue(LightVisualElement.BorderTopWidthProperty, ValueResetFlags.Local);

                    cell.ResetValue(LightVisualElement.BorderLeftColorProperty, ValueResetFlags.Local);
                    cell.ResetValue(LightVisualElement.BorderLeftShadowColorProperty, ValueResetFlags.Local);
                    cell.ResetValue(LightVisualElement.BorderLeftWidthProperty, ValueResetFlags.Local);

                    cell.ResetValue(LightVisualElement.BorderDrawModeProperty, ValueResetFlags.Local);
                    cell.ResetValue(LightVisualElement.ZIndexProperty, ValueResetFlags.Local);
                }
            }
        }

Use attached to reproduce.

This is not an issue. Performing Begin/End update disposes of all elements along with the globally declared item. You need to check if the item is disposed of:
void ContextMenuOpening(object sender, ContextMenuOpeningEventArgs e)
{
    if (menuItem.IsDisposed)
    {
        menuItem = new RadMenuItem();
        menuItem.Text = "Custom menu item";
        menuItem.Click += menuItem_Click;
    }
    e.ContextMenu.Items.Add(menuItem);
}

To reproduce: please refer to the attached sample gif file. Note that it is important that the child template is bound at design time and the columns are automatically generated. 

Workaround: setup the ViewDefinition programmatically:  

To reproduce: run the attached sample project. 

Type "chef" in the search box and then try to sort some of the columns. Click the header cell several times and as a result the exception will occur.

Workaround:

        public class CustomGrid : RadGridView
        {
            public new object DataSource
            { 
                get
                {
                    return (object)base.DataSource;
                }
                set
                {
                    GridViewSearchRowInfo.Cancel = true;
                    Thread.Sleep(100);
                    base.DataSource = null;
                    this.EnableFiltering = false;
                    base.DataSource = value;
                    this.EnableFiltering = true;
                    base.CurrentRow = null; 
                }
            }
        }