There was a bug in the Visual Studio Extensions for version R1 2020 where the Upgrade wizard would set the Lightweight render mode as the default, instead of Classic. You can verify that by using the script from Determine mixed RenderModes:
If this is the case, please remove the following line from all the appSettings sections in the web.config files you have where you did not intentionally set this:
<add key="Telerik.Web.UI.RenderMode" value="lightweight"/>
If the issue persists, try to downgrade to the old version and check if the behavior changes.
If there are any other changes you have made in your code apart from upgrading the Telerik version, please prepare a simple example with a simple Splitter and Menu and attach them in an official support ticket. That would allow us to run and investigate your exact scenario and provide more accurate and specific suggestions to resolve this issue.
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