Hello Chad,
The SelectionChanged event is not fired since the selection never got changed.
After setting the IsSelected = false for the AddedItems, the respective C1TreeViewItem never got removed from the collection and still exists in the AddedItems collection, therefore, on selecting same node which you selected previously the event is not raised.
You only see the selection shifting to a different node, by setting the IsSelected = false/true for a particular C1TreeViewItem, but it is not cleared from the Addeditems/RemovedItems collection.
I have escalated it to the concerned team for further review.
Will update you as soon as I get any update from their side regarding the same.
Regards,
Reema