Beats me, but upon exporting as .map again, that one face forms a back side, and turns into a cubed box
yea this is really confusing me as well. i am trying hard to figure out how to get this to work, if badspot is even doing it like that or it is just the exporter. I originally though all the outside walls connected to the interior creating some huge brush mess. Then I though that they were flat faces, but now i am leaning towards the first one again because when you f8 inside the walls and then f7, go in third person and it acts like a brush is there except you are just clipped inside it.
This is why i really wish the map files were actually released or these questions were answered by badspot or wedge or whoever.