I read somewhere ( I think it was in ArchiWiki) where there was a bug discovered wherein ArchiCAD had problems with merging or even Boolean operations that involved curved slanted walls and straight walls meeting at a corner. This sounds like a subset of that problem. It had to do with how ArchiCAD calculates the geometry of the "empty" space of the intersection after the cleanup.
In any case I'm not certain if they ever resolved the issue (I'll try and dig up the article for you again), but my guess is that you may have to do it mechanically, or rather yourself. My suggestion is that rather than having the 2 walls meet so that AC tries to clean them up, bring the straight wall as close as possible to the curve wall without the reference lines touching, and then using a separate element (in this case I would imagine a roof with the same slope as the slant angle as your wall) either trim the straight wall or SEO subtract it and hide the operator as appropriate and make sure you adjust the Wall priorities functions of both walls to complete the "cleanup".
Rubia Torres
