OK, that's why we need VIS. But the VIS compile stage is not perfect, and can't really evaluate the environment like a human, It will just make a mess of a map if doesn't get some help. This is where, among other things, detailed brushes comes into play. I'll not bother to explain how the VIS part works, I'll just try to tell you what brushes to make detailed, it's real simple. Everything, and I mean everything that is not a wall, ceiling or floor/ground should be detailed. That way the VIS part will be quick and accurate. For instance in your screenshot the pillar, the foundation, the metal pathway and the railing should be detailed. Not the bridge's underside as it blocks my view of whatever is on top of the bridge. That is all that VIS needs to know. Though VIS might still decide that it can see what is on the other side as vis is a difficult guy to please. But to fix that you'll have to look in on manual vis and I will most definite not get in to that now.
OK, how will this help with you problem? Well my guess is that VIS decided that that part of you pillar cant be seen, and therefore need not be drawn. But, but that is not true I can see it you say. And rightly so, you can. However, VIS being a stupid git gets confused when there are lots of small brushes like in you screenshot. He'll push he's ugly head right up to one of the railing bars and claim that he can't see nothing. Well, To explain why we make stuff detailed, it's simply that detailed brushes are not seen by vis and can therefore not confuse him. This will then hopefully bring that elusive brush face out into the open.
I hope this is of some help, It was fun to write in any case.
