Workaround:
Private Sub radGridView1_CellFormatting(sender As Object, e As Telerik.WinControls.UI.CellFormattingEventArgs)
    If e.CellElement.IsPinned Then
        If e.CellElement.RowElement.DrawFill Then
            e.CellElement.DrawFill = False
        Else
            e.CellElement.ResetValue(LightVisualElement.DrawFillProperty, ValueResetFlags.Local)
        End If
    End If
End Sub

When UseCompatibleTextRendering property is set to false, the data cells overlaps the row header cells when horizontal scrolling is performed.

Here is how this issue can be worked around:
void radGridView1_CellEditorInitialized(object sender, GridViewCellEventArgs e) { ((RadMultiColumnComboBoxElement)e.ActiveEditor).AutoSizeDropDownToBestFit = true; ((RadMultiColumnComboBoxElement)e.ActiveEditor).DropDownAnimationEnabled = false; }