We have managed to reproduce the described issue with the Windows 8 theme. We have located the issue and the fix will be available with the internal build next Monday. For now I can suggest you a work around which is to modify the ClockItem
ControlTemplate in order to change the ContentPresenter named Content to ContentControl. I have attached a sample project which demonstrates the work around.
Hope this helps. I have also updated your Telerik points for your involvement.