I have tried to reproduce the issue locally and on our on-line demos but with no success. Here is a video of my test. Please review it and try to elaborate what should be done step by step in order to observe it so we can be more helpful.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed