Ah, that explains it. The an area portal defines an area (it splits it into 2 areas). And an occluder may not touch both. Its a similar thing to an area portal leak.I know this is an old post, but I have an answer for this issue.
I've been working on a map and I got this issue after creating some areaportal brushes. I tried everything but nothing have worked, until I removed these recently created areaportal brushes, which fixed the issue.
edit: wait, that's like, exactly what booj said