Storyline 360 trouble with drag and drop changing which object is overtop of other objects.

The support engineer confirmed that this behavior is indeed a bug and they added it to their list of bugs to be worked on.  I adapted my activity to use a discard pile of face up cards that can be drawn from which forces the learner to drag every card which then makes that card on top, and the activity works.  It isn't the most elegant solution and adds more failure points (they could cheat by dragging a card back to the draw pile starting location), but it works well enough for now.