Anyone else having a problem where, under certain circumstances in 6.11.4, map entities are disappearing when moved?

I have a level -- your standard hollowed out box -- and when my map entities dip below z=0, they disappear from view. This did not occur under 6.00, and only now happens under certain circumstances, (which I'm trying to determine now). I was wondering if anyone else had run into this, and had come across a solution.

Thanks,
Tesanders

The only time I've ever had entities disappear is when a camera enters the level geometry. Every entity disappears but comes back once the camera is back in play.

Curiously, this is the opposite -- if, once my map entity vanishes, I take the camera into level geometry, that map entity will re-appear.