Hi Przemas, I can confirm that's a bug in the surface/surface intersector in the geometry library. I'll investigate it to see if I can figure out what's going wrong. Thanks for posting the .3dm file so I could reproduce it over here.
It hit me when I've been running a workshop on using digital tools to create jewelry - I've suggested MoI trial to the participants and we've been working on a simple signet ring with sweeps. In the end I had to tweak my plan a little and had to use single rail as at first I didn't noticed what's wrong.
