I can reproduce this in the Diagram Editor Example.
Place 20 rectangular shapes on the drawing.
Select 19 of them and start to drag move.
All shapes will be shown on preview layer and I will get BoundsChanged events on them.
Select all 20 shapes and start to drag move.
It doesn't show the individual shapes in the preview layer, just an outer bounding rect.
It also doesn't generate BoundsChanged events for the individual shapes.

I assume this is an optimization limit.  Is there some way of controlling that?  I looked on the MoveTool and PreviewLayer but I couldn't see anything.  

We'd like that limit to be much higher.  Even with many shapes selected, we sant to see them in the move preview and get BoundsChanged events on them.

Thanks in advance,
Jason