Areaportals are non-functional unless they're separating two areas.
What this means practically is that if you can trace a path from one side of an areaportal to the other side, it's an "areaportal leak" and can cause this issue.
You can visualise this path by going to Map > Load Pointfile after a compile that has an areaportal leak.
A common cause of areaportal leaks is func_detail geometry - the path check from one side of an areaportal to the other side will go through func_detail, props, and displacements as though they weren't even there.
Since that arch in your picture is almost certainly a func_detail, and so is the curved wall next to it, it might be the cause of the broken areaportal.