The near clipping plane is something used internally by the 3D engine to help clip and project polygons. I kind of regret bringing this subject up since this is an internal 'engine thing' and not something you can change (nor should you really want to). If you want to learn more about clipping planes, projection, and the 'view fulcrum' I can suggest a couple of books on these subjects (some of them are even understandable to people without a degree in rocket science ).

The reason I mentioned this is that the effect you are describing sounds like it might be related to the near clipping plane. But only JCL could really test if this is true or not.

Even if we do find a fix for this I would still encourage you to use map entities rather than such a large mdl. On weaker 3D cards you will get a "triangle too big for 3d card"

JCL:
The problem appears on the 4.23 standard engine (I can forward you the files if you like). It looks like A4 is clipping large polygons when part of them are extending past the near clipping plane.