Without your implementation I cannot be sure what is causing the reported behavior. However, keep in mind that when the UI Virtualization of RadTreeView is enabled only the containers of the RadTreeViewItems that are in the view port are generated and added in the visual tree. In other words if the item is not visible on the screen it does not exist in the visual tree of the application. In some scenarios this leads to a different behavior than the case where the tree is not virtualized.
If you give me more information about your implementation, like code or a an isolated project, I can test it on my side and investigate what is causing this behavior.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.