To reproduce :
public partial class RadForm1 : Telerik.WinControls.UI.RadForm
{
    public RadForm1()
    {
        InitializeComponent();
        radGridView1.DataSource = GetTable();
    }

    private void radButton1_Click(object sender, EventArgs e)
    {
        var exporter = new GridViewPdfExport(radGridView1);
        exporter.FileExtension = "pdf";
        exporter.ShowHeaderAndFooter = true;
        exporter.LeftFooter = GridViewPdfExport.DatePrintedString;
        exporter.FitToPageWidth = true;
        exporter.PageMargins = new Padding(20, 15, 10, 10);
        exporter.RunExport(@"C:\Users\dkaramfi\Desktop\test123.pdf", new PdfExportRenderer());

    }

    static DataTable GetTable()
    {

        DataTable table = new DataTable();
        table.Columns.Add("Dosage", typeof(int));
        table.Columns.Add("Drug", typeof(string));
        table.Columns.Add("Name", typeof(string));
        table.Columns.Add("Name1", typeof(string));
        table.Columns.Add("Name2", typeof(string));
        table.Columns.Add("Name3", typeof(string));
        table.Columns.Add("Name4", typeof(string));


        table.Rows.Add(50, "Enebrel", "Sam", "Sam1", "Sam2", "Sam4", "Sam4");
        table.Rows.Add(25, "Indocin", "David");
        table.Rows.Add(50, "Enebrel", "Sam");
        table.Rows.Add(10, "Hydralazine", "Christoff");
        table.Rows.Add(21, "Combivent", "Janet");
        table.Rows.Add(100, "Dilantin", "Melanie");
        return table;
    }
}

Workaround:
Leave the default margins.

To reproduce:

Sub New()
    InitializeComponent()

    ThemeResolutionService.ApplicationThemeName = "Breeze"

    Dim dt As New DataTable
    dt.Columns.Add("Id", GetType(Integer))
    dt.Columns.Add("ParentId", GetType(Integer))
    dt.Columns.Add("Name", GetType(String))

    For index = 1 To 5
        dt.Rows.Add(index, 0, "Parent" & index)
    Next
    Dim rand As New Random
    For index = 6 To 25
        dt.Rows.Add(index, rand.Next(1, 6), "Parent" & index)
    Next
    Me.RadGridView1.Relations.AddSelfReference(Me.RadGridView1.MasterTemplate, "Id", "ParentId")
    Me.RadGridView1.DataSource = dt
    Me.RadGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill

    Me.RadGridView1.AutoSizeRows = True
End Sub

To reproduce:
 private void Form1_Load(object sender, EventArgs e)
 { 
     this.productsTableAdapter.Fill(this.nwindDataSet.Products); 
     this.categoriesTableAdapter.Fill(this.nwindDataSet.Categories);

     this.radGridView1.DataSource = this.productsBindingSource;
     GridViewMultiComboBoxColumn col = new GridViewMultiComboBoxColumn();
     col.DataSource = this.categoriesBindingSource;
     col.MinWidth = 200;
     col.DisplayMember = "CategoryName";
     col.ValueMember = "CategoryID"; 
     this.radGridView1.Columns.Add(col);
     this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
     this.radGridView1.CellValueChanged += radGridView1_CellValueChanged;
 }

 private void radGridView1_CellValueChanged(object sender, GridViewCellEventArgs e)
 {
     RadMessageBox.Show("CellValueChanged. Value >> " + e.Value.ToString());
 }

To reproduce:

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Me.ProductsTableAdapter.Fill(Me.NwindDataSet.Products)
    Me.CategoriesTableAdapter.Fill(Me.NwindDataSet.Categories)

    RadGridView1.AutoGenerateHierarchy = True
    RadGridView1.DataSource = Me.NwindDataSet
    RadGridView1.DataMember = "Categories"

    Me.RadGridView1.MasterTemplate.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill
    Me.RadGridView1.MasterTemplate.Templates(0).AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill

    Me.RadGridView1.UseScrollbarsInHierarchy = True
End Sub

Private Sub RadGridView1_ViewCellFormatting(sender As Object, e As CellFormattingEventArgs) Handles RadGridView1.ViewCellFormatting
    If e.Row.HierarchyLevel > 0 Then
        e.CellElement.TableElement.RowHeaderColumnWidth = 100
    End If
End Sub

To reproduce:

private void Form1_Load(object sender, EventArgs e)
{ 
    this.suppliersTableAdapter.Fill(this.nwindDataSet.Suppliers); 
    this.productsTableAdapter.Fill(this.nwindDataSet.Products);

    radGridView1.DataSource = nwindDataSet.Suppliers;
    radGridView1.BestFitColumns(BestFitColumnMode.AllCells);
    GridViewTemplate template = CreateTemplate(this.productsBindingSource);
    template.DataSource = nwindDataSet.Products;
    radGridView1.MasterTemplate.Templates.Add(template);

    GridViewRelation relation = new GridViewRelation(radGridView1.MasterTemplate);
    relation.ChildTemplate = template;
    relation.RelationName = "SuppliersProducts";
    relation.ParentColumnNames.Add("SupplierID");
    relation.ChildColumnNames.Add("SupplierID");
    radGridView1.Relations.Add(relation);
}

private GridViewTemplate CreateTemplate(BindingSource source)
{
    GridViewTemplate template = new GridViewTemplate();
    template.Caption = "TaskTemplate";
    template.AllowAddNewRow = false;
    template.EnableFiltering = true;
    FilterDescriptor filter = new FilterDescriptor();
    filter.PropertyName = "ReorderLevel";
    filter.Operator = FilterOperator.IsNotEqualTo;
    filter.Value = 25;
    template.FilterDescriptors.Add(filter);
    template.ShowFilteringRow = false;   
    template.SortDescriptors.Expression = "ProductName ASC, UnitPrice ASC";
    template.DataSource = source;
    template.BestFitColumns(BestFitColumnMode.AllCells);

    return template;
}

Workaround: set the filter after the hierarchy setup:
private void Form1_Load(object sender, EventArgs e)
{ 
    this.suppliersTableAdapter.Fill(this.nwindDataSet.Suppliers); 
    this.productsTableAdapter.Fill(this.nwindDataSet.Products);

    radGridView1.DataSource = nwindDataSet.Suppliers;
    radGridView1.BestFitColumns(BestFitColumnMode.AllCells);
    GridViewTemplate template = CreateTemplate(this.productsBindingSource);
    template.DataSource = nwindDataSet.Products;
    radGridView1.MasterTemplate.Templates.Add(template);

    GridViewRelation relation = new GridViewRelation(radGridView1.MasterTemplate);
    relation.ChildTemplate = template;
    relation.RelationName = "SuppliersProducts";
    relation.ParentColumnNames.Add("SupplierID");
    relation.ChildColumnNames.Add("SupplierID");
    radGridView1.Relations.Add(relation);
    
    FilterDescriptor filter = new FilterDescriptor();
    filter.PropertyName = "ReorderLevel";
    filter.Operator = FilterOperator.IsNotEqualTo;
    filter.Value = 25; 
    template.FilterDescriptors.Add(filter);
}

private GridViewTemplate CreateTemplate(BindingSource source)
{
    GridViewTemplate template = new GridViewTemplate();
    template.Caption = "TaskTemplate";
    template.AllowAddNewRow = false;
    template.EnableFiltering = true;
  
    template.ShowFilteringRow = false;   
    template.SortDescriptors.Expression = "ProductName ASC, UnitPrice ASC";
    template.DataSource = source;
    template.BestFitColumns(BestFitColumnMode.AllCells);
    
   
    return template;
}

Please refer to the attached screenshot and sample video.

Workaround:
public class CustomGrid : RadGridView
{
    public override string ThemeClassName  
    { 
        get 
        { 
            return typeof(RadGridView).FullName;  
        }
    }

    protected override void OnKeyPress(KeyPressEventArgs e)
    {
        this.BeginEdit();
        if (this.GridViewElement.ActiveEditor is RadDropDownListEditor)
        {
            string symbol = e.KeyChar.ToString();
            RadDropDownListEditor editor = this.GridViewElement.ActiveEditor as RadDropDownListEditor;
            RadDropDownListEditorElement element = editor.EditorElement as RadDropDownListEditorElement;

            if ((element.AutoCompleteMode & AutoCompleteMode.Suggest) == AutoCompleteMode.Suggest)
            {
                element.EditableElementText += symbol;
                element.EditableElement.SelectionStart = 1;
                element.EditableElement.SelectionLength = 0;
            }
        }
        base.OnKeyPress(e);
    }
}

To reproduce:

private void Form1_Load(object sender, EventArgs e)
{
    this.productsTableAdapter.Fill(this.nwindDataSet.Products);
    this.categoriesTableAdapter.Fill(this.nwindDataSet.Categories);
    
    radGridView1.AllowSearchRow = true;
    radGridView1.DataSource = nwindDataSet.Categories;
    radGridView1.MasterTemplate.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
    radGridView1.UseScrollbarsInHierarchy = true;

    GridViewTemplate template = new GridViewTemplate();
    template.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
    template.DataSource = nwindDataSet.Products;
    radGridView1.MasterTemplate.Templates.Add(template);

    GridViewRelation relation = new GridViewRelation(radGridView1.MasterTemplate);
    relation.ChildTemplate = template;
    relation.RelationName = "CategoriesProducts";
    relation.ParentColumnNames.Add("CategoryID");
    relation.ChildColumnNames.Add("CategoryID");
    radGridView1.Relations.Add(relation); 
}

Workaround:
private void radGridView1_CurrentRowChanged(object sender, CurrentRowChangedEventArgs e)
{
    if (this.radGridView1.CurrentRow != null)
    {
        if (this.radGridView1.CurrentRow.HierarchyLevel > 0)
        {
            tableElement.ScrollToRow((GridViewHierarchyRowInfo)(this.radGridView1.CurrentRow).Parent);
             this.radGridView1.TableElement.EnsureRowVisible((GridViewHierarchyRowInfo)(this.radGridView1.CurrentRow).Parent);
            tableElement.ScrollToRow(this.radGridView1.CurrentRow);
            tableElement.EnsureRowVisible(this.radGridView1.CurrentRow);
        }
    }
}

To reproduce:
Search a specific text by focusing the search box programmatically and the using the SendKeys method:
private void radButton1_Click(object sender, EventArgs e)
{
    GridSearchCellElement searchCell = radGridView1.TableElement.GetCellElement(radGridView1.MasterView.TableSearchRow, null) as GridSearchCellElement;
    if (searchCell != null)
    {
        searchCell.SearchTextBox.Focus();
        searchCell.SearchTextBox.Text = string.Empty;
        SendKeys.Send("t");
        SendKeys.Send("e");
        SendKeys.Send("s");
        SendKeys.Send("t");
    }
}

Workaround: 
Repeat the search in the SearchProgressChanged event:

radGridView1.MasterView.TableSearchRow.SearchProgressChanged += TextationSearchProgressHandler;

protected void TextationSearchProgressHandler(object sender, SearchProgressChangedEventArgs e)
{
    if (e.SearchFinished && null != radGridView1.TableElement)
    {
        GridSearchCellElement searchCell = radGridView1.TableElement.GetCellElement(radGridView1.MasterView.TableSearchRow, null) as GridSearchCellElement;
        if (searchCell != null
             && searchCell.SearchTextBox.TextBoxItem.Text != e.SearchCriteria)
        {
            radGridView1.MasterView.TableSearchRow.Search(searchCell.SearchTextBox.TextBoxItem.Text);
        }
    }
}

To reproduce:

public class Item
{
    public int Id { get; set; }

    public string Name { get; set; }

    public DateTime Date { get; set; }

    public Item(int id, string name, DateTime date)
    {
        this.Id = id;
        this.Name = name;
        this.Date = date;
    }
}
public Form1()
{
    InitializeComponent();
    List<Item> items = new List<Item>();
    for (int i = 0; i < 10; i++)
    {
        items.Add(new Item(i,"Item" + i,DateTime.Now.AddHours(i)));
    }
    this.radGridView1.DataSource = items;
    this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
}

private void Form1_Load(object sender, EventArgs e)
{
    System.Threading.Thread.CurrentThread.CurrentCulture = new System.Globalization.CultureInfo("en-GB");
    this.radGridView1.Columns["Date"].ExcelExportFormatString = "M/d/yyyy h:mm tt";
    this.radGridView1.Columns["Date"].ExcelExportType = DisplayFormatType.Custom;

    GridViewSpreadExport spreadExporter = new GridViewSpreadExport(this.radGridView1);
    spreadExporter.ExportVisualSettings = true;
    SpreadExportRenderer exportRenderer = new SpreadExportRenderer();
    string fileName = @"..\..\exportedFile" + DateTime.Now.ToLongTimeString().Replace(":", "_") + ".xlsx";
    spreadExporter.RunExport(fileName, exportRenderer);
    Process.Start(fileName);
}

Workaround: use a custom BaseGridBehavior

public Form1()
{
    InitializeComponent();

    this.radGridView1.GridBehavior = new MyBaseGridBehavior();
}

public class MyBaseGridBehavior : BaseGridBehavior
{
    public override bool ProcessKey(KeyEventArgs keys)
    {
        if (keys.Control && this.GridControl.CurrentColumn == null)
        {
            return false;
        }

        return base.ProcessKey(keys);
    }
}

To reproduce:
- Add some rows to a grid.
- Sort the rows.
- Delete a row.
- The current row is not the next row.

Workaround:
Dim t As Test = RadGridView1.CurrentRow.DataBoundItem
Dim index As Integer = Me.RadGridView1.ChildRows.IndexOf(Me.RadGridView1.CurrentRow)
datasource.Remove(t)
Me.RadGridView1.CurrentRow = Me.RadGridView1.ChildRows(index)

To reproduce:

1. Run the attached application. 

2. Click into the cell in Column One and expand the combo box

3. While the combo box is still expanded, right click on any header cell in the grid

Step 3 should result in a NullReferenceException

Workaround:
Friend Class MyGridHeaderCellElement
    Inherits GridHeaderCellElement
 
    Public Sub New(ByVal col As GridViewColumn, ByVal row As GridRowElement)
        MyBase.New(col, row)
    End Sub
    Protected Overrides Sub ShowContextMenu()
        If Me.ViewTemplate IsNot Nothing Then
            MyBase.ShowContextMenu()
        End If
    End Sub
    Protected Overrides ReadOnly Property ThemeEffectiveType() As Type
        Get
            Return GetType(GridHeaderCellElement)
        End Get
    End Property
End Class

Private Sub radGridView1_CreateCell(ByVal sender As Object, ByVal e As GridViewCreateCellEventArgs)
    If Object.ReferenceEquals(e.CellType, GetType(GridHeaderCellElement)) Then
        e.CellType = GetType(MyGridHeaderCellElement)
    End If
 
End Sub

To reproduce:

public Form1() 
{
    InitializeComponent();

    DataTable dt = new DataTable();
    for (int i = 0; i < 5; i++)
    {
        dt.Columns.Add("Col"+i);
    }
    for (int i = 0; i < 2000; i++)
    {
        DataRow row = dt.NewRow();
        foreach (DataColumn col in dt.Columns)
        {
            row[col.ColumnName] = "Data" + i + "." + dt.Columns.IndexOf(col);
        }
        dt.Rows.Add(row);
    }
    this.radGridView1.DataSource = dt;
    this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
}

private void radButton1_Click(object sender, EventArgs e)
{
    RadPrintPreviewDialog dialog = new RadPrintPreviewDialog();
    dialog.Document = this.radPrintDocument1;
    dialog.ShowDialog();
}

Workaround: this.radGridView1.AutoSizeRows = true;

To reproduce:

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Me.RadGridView1.AutoGenerateColumns = False

    Dim column = New GridViewTextBoxColumn
    column.Name = "Name"
    column.FieldName = "ChildItem"
    column.DataType = GetType(CustomItem)
    column.DataTypeConverter = New CustomItemTypeConverter()

    Me.RadGridView1.Columns.Add(column)
    Me.RadGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill

    Dim objects =
        {
            New CustomContainer With {.ChildItem = New CustomItem With {.Name = "A"}},
            New CustomContainer With {.ChildItem = New CustomItem With {.Name = "B"}},
            New CustomContainer With {.ChildItem = New CustomItem With {.Name = "C"}}
        }

    Me.RadGridView1.DataSource = objects
End Sub

Private Sub RadGridView_EditorRequired(sender As Object, e As EditorRequiredEventArgs) Handles RadGridView1.EditorRequired
    e.EditorType = GetType(GridSpinEditor)
End Sub

Class CustomContainer
    Public Property ChildItem As CustomItem
End Class

Class CustomItem
    Public Property Name As String
End Class

Class CustomItemTypeConverter
Inherits TypeConverter

    Public Overrides Function CanConvertFrom(context As ITypeDescriptorContext, sourceType As Type) As Boolean
        If sourceType = GetType(Decimal) Then
            Return True
        End If

        Return MyBase.CanConvertFrom(context, sourceType)
    End Function

    Public Overrides Function CanConvertTo(context As ITypeDescriptorContext, destinationType As Type) As Boolean
        If destinationType = GetType(Decimal) OrElse destinationType = GetType(String) OrElse destinationType = GetType(CustomItem) Then
            Return True
        End If

        Return MyBase.CanConvertTo(context, destinationType)
    End Function

    Public Overrides Function ConvertTo(context As ITypeDescriptorContext, culture As System.Globalization.CultureInfo, _
                                        value As Object, destinationType As Type) As Object
        If TypeOf value Is CustomItem Then
            Dim customValue = DirectCast(value, CustomItem)

            If destinationType = GetType(Decimal) Then
                Return Microsoft.VisualBasic.AscW(customValue.Name.Chars(0))
            ElseIf destinationType = GetType(String) Then
                Return customValue.Name
            ElseIf destinationType = GetType(CustomItem) Then
                Return customValue
            Else
                Return MyBase.ConvertTo(context, culture, value, destinationType)
            End If
        End If

        Return MyBase.ConvertTo(context, culture, value, destinationType)
    End Function

    Public Overrides Function ConvertFrom(context As ITypeDescriptorContext, culture As System.Globalization.CultureInfo, value As Object) As Object
        If TypeOf value Is Decimal Then
            Dim decValue = DirectCast(value, Decimal)
            Dim intValue = CInt(decValue)
            Dim charValue = Microsoft.VisualBasic.ChrW(intValue)

            Return New CustomItem With {.Name = New String(charValue, 1)}
        End If

        Return MyBase.ConvertFrom(context, culture, value)
    End Function

End Class


Workaround:

Public Class MyEditor
    Inherits GridSpinEditor

    Public Overrides ReadOnly Property IsModified() As Boolean
        Get
            If Me.originalValue Is Nothing Then
                Return (Me.Value IsNot Nothing AndAlso Me.Value <> DBNull.Value)
            End If

            If Me.Value Is Nothing Then
                Return (Me.originalValue IsNot Nothing AndAlso Me.originalValue <> DBNull.Value)
            End If
            Dim column As GridViewDataColumn = (DirectCast(Me.OwnerElement, GridDataCellElement)).ColumnInfo
            If column.DataTypeConverter IsNot Nothing AndAlso column.DataTypeConverter.CanConvertTo(Me.OwnerElement, GetType(Decimal)) Then
                Return Not column.DataTypeConverter.ConvertTo(Me.OwnerElement, column.FormatInfo, Me.originalValue, GetType(Decimal)).Equals(Convert.ToDecimal(Me.Value))
            End If

            Return Not Convert.ToDecimal(Me.originalValue).Equals(Convert.ToDecimal(Me.Value))
        End Get
    End Property
End Class


Private Sub RadGridView_EditorRequired(sender As Object, e As EditorRequiredEventArgs) Handles RadGridView1.EditorRequired
    e.EditorType = GetType(MyEditor)
End Sub

All event handlers should be made virtual.
All controls should be public or have properties.

To reproduce:

private void Form2_Load(object sender, EventArgs e) 
{ 
    this.productsTableAdapter.Fill(this.nwindDataSet.Products); 
    this.categoriesTableAdapter.Fill(this.nwindDataSet.Categories);

    this.radGridView1.AutoGenerateHierarchy = true;
    this.radGridView1.DataSource = this.nwindDataSet;
    this.radGridView1.DataMember = "Categories";
    this.radGridView1.MasterTemplate.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
    this.radGridView1.MasterTemplate.Templates.First().AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

    GridViewSummaryItem summaryItem = new GridViewSummaryItem();
    summaryItem.Name = "CategoryID";
    summaryItem.Aggregate = GridAggregateFunction.Count;
    GridViewSummaryRowItem summaryRowItem = new GridViewSummaryRowItem();
    summaryRowItem.Add(summaryItem);
    this.radGridView1.SummaryRowsTop.Add(summaryRowItem);

    GridViewSummaryItem summaryItem2 = new GridViewSummaryItem();
    summaryItem2.Name = "UnitPrice";
    summaryItem2.Aggregate = GridAggregateFunction.Sum;
    GridViewSummaryRowItem summaryRowItem2 = new GridViewSummaryRowItem();
    summaryRowItem2.Add(summaryItem2);
    this.radGridView1.MasterTemplate.Templates.First().SummaryRowsTop.Add(summaryRowItem2); 
}

private void SetNumberFormat()
{
    this.radGridView1.Columns["CategoryID"].FormatString = "{0:F4}";
    this.radGridView1.SummaryRowsTop[0][0].FormatString = "{0:F4}";

    this.radGridView1.MasterTemplate.Templates.First().Columns["UnitPrice"].FormatString = "{0:F4}";
    this.radGridView1.MasterTemplate.Templates.First().SummaryRowsTop[0][0].FormatString = "{0:F4}";
}

private void radButton1_Click(object sender, EventArgs e)
{
    SetNumberFormat();
}

Workaround: Clear and add back the summary rows when a value in the child template is changed. 
private void radButton1_Click(object sender, EventArgs e)
{
    this.radGridView1.MasterTemplate.Templates.First().SummaryRowsTop.Clear();

    GridViewSummaryItem summaryItem2 = new GridViewSummaryItem();
    summaryItem2.Name = "UnitPrice";
    summaryItem2.Aggregate = GridAggregateFunction.Sum;
    GridViewSummaryRowItem summaryRowItem2 = new GridViewSummaryRowItem();
    summaryRowItem2.Add(summaryItem2);
    this.radGridView1.MasterTemplate.Templates.First().SummaryRowsTop.Add(summaryRowItem2); 

    SetNumberFormat();
}

Workaround: use custom editor element

private void radGridView1_EditorRequired(object sender, EditorRequiredEventArgs e)
{
    if (e.EditorType == typeof(RadMultiColumnComboBoxElement))
    {
        e.EditorType = typeof(MyRadMultiColumnComboBoxElement);
    }
}

public class MyRadMultiColumnComboBoxElement : RadMultiColumnComboBoxElement
{
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadMultiColumnComboBoxElement);
        }
    }

    protected override void ProcessKeyDown(object sender, KeyEventArgs e)
    {
        base.ProcessKeyDown(sender, e);

        FieldInfo fi = this.GetType().BaseType.BaseType.GetField("oldTextValue", BindingFlags.Instance | BindingFlags.NonPublic);
        fi.SetValue(this, this.textBox.Text.Substring(0, this.textBox.SelectionStart));
    }
}

To reproduce:

1. Add a RadGridView with a GridViewMultiComboBoxColumn. Enable the auto filter functionality for this column and add an appropriate FilterDescriptor to the RadMultiColumnComboBoxElement.
2. Using the keyboard arrows only (no mouse), navigate to the GridViewMultiComboBoxColumn.
3. Type "ba" by using the keyboard. The "b" is lost and only the "a" gets to the filter. I expect the filter to show "ba".

Workaround: use custom row behavior

public Form1()
{
    InitializeComponent();
    //register the custom row  behavior
    BaseGridBehavior gridBehavior = radGridView1.GridBehavior as BaseGridBehavior;
    gridBehavior.UnregisterBehavior(typeof(GridViewDataRowInfo));
    gridBehavior.RegisterBehavior(typeof(GridViewDataRowInfo), new CustomGridDataRowBehavior());
}

public class CustomGridDataRowBehavior : GridDataRowBehavior
{
    protected override bool ProcessAlphaNumericKey(KeyPressEventArgs keys)
    {
        bool result = base.ProcessAlphaNumericKey(keys);
        if (this.IsInEditMode &&
            (this.BeginEditMode == RadGridViewBeginEditMode.BeginEditOnKeystroke ||
             this.BeginEditMode == RadGridViewBeginEditMode.BeginEditOnKeystrokeOrF2))
        {
            if (this.GridViewElement.ActiveEditor is RadMultiColumnComboBoxElement)
            {
                this.GridViewElement.ActiveEditor.Value = keys.KeyChar;
                if (this.GridViewElement.IsInEditMode)
                {
                    RadMultiColumnComboBoxElement mccb = (RadMultiColumnComboBoxElement)this.GridViewElement.ActiveEditor;
                    RadTextBoxItem textBoxItem = mccb.TextBoxElement.TextBoxItem;
                    textBoxItem.Clear();
                    textBoxItem.Text =  keys.KeyChar.ToString();
                    textBoxItem.SelectionStart = 1;
                    textBoxItem.SelectionLength = 0;
                }

                return true;
            }
        }
        return result;
    }
}

To reproduce:
public abstract class Base
{
    public abstract string Name { get; }
}

public class Child : Base
{
    public override string Name
    {
        get { return "Child"; }
    }
}

public class Child2 : Base
{
    public override string Name
    {
        get { return "Child2"; }
    }
}

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

        var list = new List<Base>();
        list.Add(new Child());
        list.Add(new Child());
        list.Add(new Child2());

        radGridView1.DataSource = list.ToArray();
    }
}

Workaround:
 radGridView1.DataSource = list;