TreeView: PreviewUnchecked doesn't stop unchecking of parent node when handled

If we have a TreeView with its "IsTriStateMode" property set to "True" and handle PreviewUnchecked event it is still possible to "uncheck" a root node.
The root node should have at least 2 successors and only one of them is checked. And the parent node is in "intermediate" (not checked/not unchecked) state. If we handle PreviewUncheck event, clicking of the parent node result in clearing (unchecking) of this node and all successors. This should not happen.
There is a sample project attached.

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.

In DataBound trees you can use the following approach =>  http://docs.telerik.com/devtools/wpf/controls/radtreeview/how-to/howto-tri-state-mvvm.html