It is not quite clear if the described behavior relates to Sitefinity administration or to the public site. It will be very helpful if you explain in details the steps for reproducing the issue. Also, please send us a simple example (a user control nesting another one) demonstrating the problem to debug it locally.
You could also try to turn off the cache for the corresponding page and check if the problem still persists.
Thanks for your cooperation in advance.
the Telerik team
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