Just a stupid idea: Could it be that it has something to do with the Guardian Boundary Editor? We both use that tool and have the same problems. I have often had it that the field surface went beyond the blue borders, mostly with the elevators.
I will try it out but it sould not matter. In the Richard's video you can see how far beyond guardian boundaries does the corridor reach. And he said that the area in menu is the same as safe area (which is also guardian boundary).
That said, manipulating guardian boundary with GBE may still make it easier to repro.