At the moment when any of the properties are changed, e.g. the frozen panes, the UI is not notified and you have to export and import the file in order for the UI to pick them up. 


Workaround: There are several workarounds:

1. Change the active worksheet and return to the old one

2. Export and import the document

3. Change the workbook of the RadSpreadsheet to a new one, and return the old one after that
4. Freeze the active worksheet using the ActiveWorksheetEditor.FreezePanes() method and freeze all others using the ViewState:
RadWorksheetEditor editor = this.radSpreadsheet.ActiveWorksheetEditor;
editor.FreezePanes(new CellIndex(3,3));
foreach (var worksheet in this.radSpreadsheet.Workbook.Worksheets)
{
    worksheet.ViewState.FreezePanes(5, 5);
}