The problem is due to a well known disadvantage of MS AJAX that it could not work properly with history.back() and breaks the back button functionality of the browser. The browser Back and Forward functionality is only available when a full postback has occurred and that is the reason for them not to remember the last page state after ajax request. You can also refer to this help article
for more information about browsers history with Ajax.
However, in order to enable browser back and forward buttons with ajax you can use the ScriptManager History points for that purpose as demonstrated in the attached project.
All the best,
the Telerik team
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