The current worksheet is not updated if the workbook is set while the control is disabled.

Workaround: 

private void RadButton_Click( object sender, RoutedEventArgs e )
{
    radSpreadsheet.IsEnabled = false;
  
    Workbook spreadsheetWorkbook =
        new XlsxFormatProvider( ).Import( File.ReadAllBytes( @"..\..\Book2.xlsx" ) );
  
    radSpreadsheet.Workbook = new Workbook( );
  
    Dispatcher.InvokeAsync( ( ) => radSpreadsheet.Workbook = spreadsheetWorkbook );
  
    radSpreadsheet.IsEnabled = true;
}