The TreeView doesn't update the TriState of the parent item when an item is dragged and dropped.

In databound scenario the built-in checkbox and the tri-state mode are considered not supported and users are prompted to use checkbox in itemtemplate with 3state logic in the viewmodels.
http://docs.telerik.com/devtools/wpf/controls/radtreeview/how-to/howto-tri-state-mvvm.html 

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.