When using animations on refreshing the layout of a RadDiagram (LayoutSettings.AnimateTransitions = true, then calling Diagram.Layout or Diagram.LayoutAsync), if you have a RadDiagramThumbnail "synchronized" with the RadDiagram... well, the synchronization does not happen automatically (just this I believe is a miss on the part of the Diagramming Framework) and, if you force it with a call to RefreshThumbnail(), since both the Layout and the RefreshThumbnail processes run on the UI thread they seem to collide and the result is the thumbnail isn't correctly refreshed either.
Obviously, best would be for those theoretically-synchronized controls to really work in-sync even when animations are on (they seem to work ok for animations off). But at a minimum if the RadDiagram had an event that was thrown when a Layout operation has finalized, we could listen to that event and call the RefreshThumbnail then.
Update: RadDiagram provides public DiagramLayoutComplete event. This event is available since Q2 2012.