When the algorithm is creating scrap outline, it goes from end of one line to the nearest end of second line. In your case, when it comes to the end of debris line, it connects the second line, but on the wrong end. Than triangle appears.

When you will connect the end of debris line with invisible wall to the correct wall end on the other side of the passage, your problem should be fixed (at least on my machine is). Matapost also complains about this - that scrap outline intersect it self.

I know, that this algorithm can be improved, but... But at least, I will put it in the TODO list.