We've posted a reply in the support thread you started and it would be best to continue the communication there.
Also, in case anyone else encounters the same issue I will post the suggested solution here as well:
By design the Hierarchy's content is re-loaded once the collection is Reset (through the SortDescriptors collection in this case). As a workaround we would suggest you to persist the state of the selected TabItem when the TabControl is unloaded (Unloaded event is raised) and then set it back when the Loaded event is raised for it.
the Telerik team