One addiitonal data point:
When the little minimize button in the mini bar is grayed out the position of the fullscreen VM moves around. For example:
-- assume four virtual screens
-- fourth virtual screen contains a fullscreen VM
-- close the Mac lid
-- open the Mac lid (after some period of time, such as 15 minutes)
-- fullscreen is now in some other location, no longer in the fourth position
Why is the fullscreen VM moving around?