I created a sample test page based on your description, however, I was not able to reproduce the problem you mention. Please, find my test page attached and let me know what I am missing. I would like to request for a simple page which reproduces the problem so that I can get a closer look at your scenario and do my best to help you.
You will need to open a support ticket in order to have the right to attach files. Please, download a trial version of the RadControls for ASP.NET AJAX suite and you will have the right to open a support ticket.
the Telerik team