SnowdreamCould you create a clipbrush at a right angle to the y axis of the map and cover the non-right-angled wall with it? Might be missing the point, I'm not a mapper.
Here's an explanation of sorts. Your suggestion, the tilde one, works to prevent the bug from happening. On maps like process, however, where the oblique walls are relatively large, the resulting clip brush is too large to be reasonable. The only other fix that I could think of also has an issue.