Indeed this seem to be an odd behavior and we haven't encountered it before. However from the looks of it it doesn't seem to be caused by RadTreeView.
Do you use RadAjaxManager for the partial postback, or an UpdatePanel? Also, does the problem reproduce if you use an ordinary asp:TreeView and asp:UpdatePanel?
It would also be helpful if you sent us a runnable sample solution demonstrating the issue, and possibly a live Url, so that we can observe the issue and investigate it locally.
the Telerik team
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