I answered you in the other thread as well. As you correctly noticed this behavior is by design and it could be switched off by setting the IsContentPreserved property of the RadPaneGroup. For more information about this feature you could refer to this help article. As this feature had a bug in the official release I would suggest you to use the latest internal build. The fix will be officially released at the end of this month.
Please noticed that preserving the content will preserve it only between tab changes, but dragging a pane out will unload and reload the content. This behavior cannot be changed.
the Telerik team
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