The problem is caused by a lack of prediction in the `isect_line_segment_tri_v3` that incorrectly confirms some intersections of coplanar segments to the triangle. The solution is to use another algorithm to detect intersections. This also resulted in a slight improvement in the performance: - 1min 17sec to 1min 6sec in my test file Differential Revision: https://developer.blender.org/D8500