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