Tree is Virtualized and bind to colelction of DataItems.
On a button click, we add new Dataitem with IsInEditMode = true (bound to IsInEditMode of the RadTreeViewItem).
The new Item is in edit mode but the textBox in the ItemEditTemplate has no DataContext so no actual editing  can be performed.

Scenario casn be solved with adding the item, then switch its editmode in dispatcher:
DataItem item = new DataItem() { Name = "Item Editable" };
             
            (this.tree.ItemsSource as ObservableCollection<DataItem>).Insert(0, item);

            Dispatcher.BeginInvoke(new Action(() =>
            {
                item.IsInEditMode = true;
            }), DispatcherPriority.Loaded);

The XAML team has recently reviewed this issue and will not be addressing it as at this time the team is focusing on the bugs impacting the highest number of developers. If you have encountered this issue and it is blocking for your work please contact us through the support ticketing system with details on your setup and a reference to this item.