The problem is not in the Ajax, you can actually remove the Ajax manager, or set EnableAjax=false, and you will see that it behaves the same way.
The problem is that you reload the RichContent during click on the menu. And in case the post back is initiated from the grid, you don't add it. So if the post back is not from the menu, your grid is missing.
You need to always load the same user controls. But for example change their Visible property when they should not be visible.
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