I tried the scenario that you describe in the following way:
I expand the tree using client-side code (OnClientNodeclicking event) and at the same time use a server-side event (OnNodeClick). In the case when I use both events the page will inevitably postback and the expanded nodes will not be visible. However, when I use each of the events separately both work fine. To avoid this behaviour you can explicitly set the ExpandMode property of RadTreeNode either to "ClientSide" or to
"ServerSideCallback" as described in this help article.
Is this your scenario or am I missing something? In case this does not help, you can open a support ticket and send us a sample project demonstrating the issue.
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