I’m not going to pretend to understand how the display works behind the scenes, but here is an observation:
If I have two coincident polysurfaces, the display seems to alternate which is displayed “on top”, and is even affected by the selection of other objects. Here is a short video illustrating that:
Here is another video showing how simply selecting the objects can alter which displays “on top”:
In V5 I could control what was displayed on top with a small script that copied the “top” object in place and deleted the original.
Notice in V5 how the selection of other objects doesn’t affect the coincident polysurfaces.