I tried to reproduce the problem you described, but without success. You can try removing the RadControls from the Visual Studio toolbox and adding them again as described here
using the latest version of the Telerik.Web.UI
If the problem persists, please open a regular support ticket and send a simple, runnable project that isolates the issue so that I can inspect it locally and provide a more to the point answer.
All the best,
the Telerik team
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