To reproduce: run the attached sample project and press the "stream export" button. 

Workaround: use the GridViewSpreadExport https://docs.telerik.com/devtools/winforms/gridview/exporting-data/spread-export

        Dim spreadExporter As GridViewSpreadExport = New GridViewSpreadExport(RadGridView1)
        Dim exportRenderer As New SpreadExportRenderer()
        Dim fileName As String = "..\..\Export" & DateTime.Now.ToLongTimeString().Replace(":", "_") + ".xlsx"
        spreadExporter.RunExport(fileName, exportRenderer)

Use attached to reproduce.
- Check the filter box
- Uncheck the header checkbox.
- The cells are still visible and you cannot click on the data check boxes.

 

To reproduce:
Public Class RadForm1
    Public RadGridView1 As RadGridView

    Private Sub RadForm1_Load(sender As Object, e As EventArgs) Handles Me.Load
        CreateGrid()
        FormatGrid()
        SetDataSource()
    End Sub

    Private Sub CreateGrid()
        Try
            RadGridView1 = New RadGridView
            Me.Controls.Add(RadGridView1)
        Catch ex As Exception
            MessageBox.Show(Me, ex.Message)
        End Try
    End Sub
    Private Sub FormatGrid()

        Dim col As GridViewTextBoxColumn
        With RadGridView1
            col = New GridViewTextBoxColumn
            With col
                .FieldName = "FieldID"
                'NOTE: Comment out NullValue = "" to prevent the unhandled exception
                .NullValue = ""
            End With
            .Columns.Add(col)
        End With
    End Sub
    Private Sub SetDataSource()
        Dim table As DataTable = Nothing
        Dim row As DataRow = Nothing
        table = New DataTable("GridList")
        table.Columns.Add("FieldID", GetType(System.Guid))
        row = table.NewRow
        row("FieldID") = Guid.Empty
        table.Rows.Add(row)
        RadGridView1.DataSource = table
    End Sub
End Class

Workaround:
Comment this line 
NullValue = ""

Please refer to the attached sample project. The ResizeColumnsProportionally.gif illustrates the correct behavior of columns resizing. However, if you make a certain column with fixed size, the columns resizing is not proportional any more. The ColumnsWidthNOTAdjustedProportionally.gif demonstrates better the wrong behavior.

Workaround:  handle the SizeChanged event and adjust the columns' width property programmatically in order to obtain the desired widths.

Until the new functionality becomes available you can use the workaround solution in the attached project.

Excel does not support dates prior to 1/1/1900, however,  we could export such dates as simple strings: https://support.office.com/en-us/article/excel-specifications-and-limits-1672b34d-7043-467e-8e27-269d656771c3

How to reproduce:
 public partial class Form1 : RadForm
 {
     public Form1()
     {
         InitializeComponent();

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

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

         DateTime date = new DateTime(1850, 1, 1);
         for (int i = 0; i < 100; i++)
         {
             dt.Rows.Add(i, "Name " + i, i % 2 == 0, date.AddYears(i));
         }

         return dt;
     }
     
     FieldInfo fi;
     private void radButton1_Click(object sender, EventArgs e)
     {
         //Old Export using the ExcelML format
         this.fi = typeof(Telerik.WinControls.UI.Export.ExcelML.CellElement).GetField("_dataElement", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
         ExportToExcelML exporter = new ExportToExcelML(this.radGridView1);
         string fileName = @"..\..\data.xls";
         exporter.RunExport(fileName);

         //New export utilizing the SpreadProcessing libraries
         GridViewSpreadExport spreadExporter = new GridViewSpreadExport(this.radGridView1);

         SpreadExportRenderer exportRenderer = new SpreadExportRenderer();
         spreadExporter.RunExport(@"..\..\data.xlsx", exportRenderer);
     }
 }


Workaround: handle the CellFormatting event
public partial class Form1 : RadForm
{
    public Form1()
    {
        InitializeComponent();

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

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

        DateTime date = new DateTime(1850, 1, 1);
        for (int i = 0; i < 100; i++)
        {
            dt.Rows.Add(i, "Name " + i, i % 2 == 0, date.AddYears(i));
        }

        return dt;
    }
    
    FieldInfo fi;
    private void radButton1_Click(object sender, EventArgs e)
    {
        //Old Export using the ExcelML format
        this.fi = typeof(Telerik.WinControls.UI.Export.ExcelML.CellElement).GetField("_dataElement", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
        ExportToExcelML exporter = new ExportToExcelML(this.radGridView1);
        exporter.ExcelCellFormatting += Exporter_ExcelCellFormatting;
        string fileName = @"..\..\data.xls";
        exporter.RunExport(fileName);

        //New export utilizing the SpreadProcessing libraries
        GridViewSpreadExport spreadExporter = new GridViewSpreadExport(this.radGridView1);
         spreadExporter.CellFormatting += SpreadExporter_CellFormatting;

        SpreadExportRenderer exportRenderer = new SpreadExportRenderer();
        spreadExporter.RunExport(@"..\..\data.xlsx", exportRenderer);
    }

    private void SpreadExporter_CellFormatting(object sender, Telerik.WinControls.Export.CellFormattingEventArgs e)
    {
        if (e.GridRowIndex >= 1 && e.GridCellInfo.ColumnInfo is GridViewDateTimeColumn && ((DateTime)e.GridCellInfo.Value).Year < 1900)
        {
            Telerik.Windows.Documents.Spreadsheet.Model.CellSelection cell = e.CellSelection as Telerik.Windows.Documents.Spreadsheet.Model.CellSelection;
            cell.SetValue(e.GridCellInfo.Value.ToString());
        }
    }

    private void Exporter_ExcelCellFormatting(object sender, Telerik.WinControls.UI.Export.ExcelML.ExcelCellFormattingEventArgs e)
    {
        if (e.GridRowIndex > -1 && e.GridCellInfo.ColumnInfo is GridViewDateTimeColumn && ((DateTime)e.GridCellInfo.Value).Year < 1900)
        {
            DataElement data = new DataElement();
            data.DataItem = e.GridCellInfo.Value.ToString();
            this.fi.SetValue(e.ExcelCellElement, data);
        }
    }
}

Workaround: handle the GridViewPdfExport.CellFormatting event and apply the column`s format string

Public Class Form1
    Sub New()

        InitializeComponent()

        Me.RadGridView1.DataSource = Me.GetData()
        Me.RadGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill

        Dim decimalColumn = DirectCast(Me.RadGridView1.Columns("Money"), GridViewDecimalColumn)
        decimalColumn.DecimalPlaces = 0
        decimalColumn.FormatString = "{0:C0}"

        Dim dateTimeColumn = DirectCast(Me.RadGridView1.Columns("Date"), GridViewDateTimeColumn)
        dateTimeColumn.FormatString = "{0:D}"

    End Sub

    Private Function GetData() As Object
        Dim dataTable As New DataTable()
        dataTable.Columns.Add("Id", GetType(Integer))
        dataTable.Columns.Add("Name", GetType(String))
        dataTable.Columns.Add("Money", GetType(Decimal))
        dataTable.Columns.Add("Date", GetType(DateTime))

        For i As Integer = 0 To 999
            dataTable.Rows.Add(i, "Name " & i, i * 10, DateTime.Now.AddDays(i))
        Next

        Return dataTable
    End Function

    Private Sub RadButton1_Click(sender As Object, e As EventArgs) Handles RadButton1.Click
        Dim pdfExporter = New GridViewPdfExport(Me.RadGridView1)
        AddHandler pdfExporter.CellFormatting, AddressOf pdfExporter_CellFormatting
        Dim renderer = New PdfExportRenderer()
        Dim fileName As String = "..\..\exported-grid.pdf"

        pdfExporter.RunExport(fileName, renderer)
    End Sub

    Private Sub pdfExporter_CellFormatting(sender As Object, e As PdfExportCellFormattingEventArgs)
        If e.RowIndex > -1 Then
            e.CellElement.Text = TryCast(RadDataConverter.Instance.Format(e.Row.Cells(e.ColumnIndex).Value, GetType(String), e.Column), String)
        End If
    End Sub

End Class

To reproduce: if you pin a calculator or a hyperlink column you will notice that the cells belonging to these columns remain white. However, the cells from other column types  have a light blue/gray fill color for pinned state. 

Workaround: 
private void RadGridView1_CellFormatting(object sender, CellFormattingEventArgs e)
{
    if (e.Column.IsPinned)
    {
        e.CellElement.BackColor = Color.FromArgb(235, 244, 252);
    }
    else
    {
        e.CellElement.ResetValue(LightVisualElement.BackColorProperty, ValueResetFlags.Local);
    }
}

To reproduce: please refer to the attached gif file and sample project.

Workaround: when you set the DrawFill property to true, specify the BackColor to the desired one and reset for the rest of the cells.

private void radGridView1_CellFormatting(object sender, Telerik.WinControls.UI.CellFormattingEventArgs e)
{
    if (e.Column.Name == "ProductName" && e.Row is GridViewDataRowInfo)
    {
        if (e.CellElement.Value.ToString().Contains("C"))
        {
            e.CellElement.DrawFill = true;
            e.CellElement.BackColor = Color.Yellow;
            e.CellElement.GradientStyle = GradientStyles.Solid;
            e.CellElement.ForeColor = Color.OliveDrab;
        }
        else
        { 
            e.CellElement.ResetValue(LightVisualElement.DrawFillProperty, ValueResetFlags.Local);
            e.CellElement.ResetValue(LightVisualElement.GradientStyleProperty, ValueResetFlags.Local);
            e.CellElement.ResetValue(LightVisualElement.ForeColorProperty, ValueResetFlags.Local);  
            e.CellElement.ResetValue(LightVisualElement.BackColorProperty, ValueResetFlags.Local);  
        }
    }
    else
    { 
        e.CellElement.ResetValue(LightVisualElement.DrawFillProperty, ValueResetFlags.Local);
        e.CellElement.ResetValue(LightVisualElement.GradientStyleProperty, ValueResetFlags.Local);
        e.CellElement.ResetValue(LightVisualElement.ForeColorProperty, ValueResetFlags.Local);    
        e.CellElement.ResetValue(LightVisualElement.BackColorProperty, ValueResetFlags.Local);  
    }
}

To reproduce:
- Add a RadGridView and a button to a blank form.
- Subscribe to CellValidating event from the grid and set the cancel property of the CellValidatingEventArgs to true upon some condition.
- Add click event handler for the button and print a message in it.
- Start the application and enter some invalid data in the grid cell, then click the button.
- The code from the button's event handler is executed.

Workaround use a flag to determine when to execute the corresponding button code:
bool validating = false;

void radGridView1_CellValidating(object sender, Telerik.WinControls.UI.CellValidatingEventArgs e)
{
    if (e.Value.ToString().Length < 5)
    {
        e.Cancel = true;
        validating = true;
        e.Row.ErrorText = "Validation error!";
    }
    else
    {
        validating = false;
    }
}

private void radButton1_Click(object sender, EventArgs e)
{
    if (!validating)
    {
        Debug.WriteLine("Executed");
    }
}

To reproduce: please refer to the attached screenshot.

Workaround: add the FilterDescriptor programmatically: https://docs.telerik.com/devtools/winforms/gridview/filtering/setting-filters-programmatically-(simple-descriptors)

To reproduce:

            this.radGridView1.Columns.Add("FIRST CELL ID A");

            this.radGridView1.Rows.Add("405-55-214-41763");
            this.radGridView1.Rows.Add("405-55-214-46682");
            this.radGridView1.Rows.Add("405-55-214-46682");

            this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
            this.radGridView1.MultiSelect = true;
            this.radGridView1.SelectionMode = Telerik.WinControls.UI.GridViewSelectionMode.CellSelect;

Please refer to the attached gif file illustrating better the text moving. 

Workaround: 
        private void radGridView1_CellFormatting(object sender, Telerik.WinControls.UI.CellFormattingEventArgs e)
        {
            e.CellElement.BorderBottomWidth = 2;
            e.CellElement.BorderRightWidth = 2;
            e.CellElement.BorderTopWidth = 2;
            e.CellElement.BorderLeftWidth = 2;
        }

To reproduce:
 this.radGridView1.SummaryRowsBottom.Insert(0, summaryRowItem2);
 this.radGridView1.SummaryRowsBottom.Move(2, 0);

Workaround:
Remove all items and add them back with a spesific order. 

How to reproduce: check the attached project and video

Bind the grid to the following DataTable:

            DataTable dt = new DataTable();     
            dt.Columns.Add("X.Y"); 
            dt.Columns.Add("X");   
            
            dt.Rows.Add("Data X.Y", "Data X");

            this.radGridView1.DataSource = dt;

You will notice that the second column "X" is missing.

Note: if you change the order of adding the columns, both columns will be added.

The new mode should allow copying of single cells even when having the SelectionMode set to FullRowSelect. The attached project features a possible workaround.