I am not quite sure what may cause the behavior that you experience. I prepared a test project based on your code, but I was not able to reproduce such behavior. Could you please open a new support ticket and send us a simple project that reproduce the described issue. For you convenience I have attached my test project to the thread and you can just modify it in order to reproduce the problem. Once we have a better view over your setup we will do our best to provide a solution.
the Telerik team