Hi mir4ea, thanks for attaching the model, I could have sworn I did not see it at first!

So it looks like all that I wrote above was incorrect, you do have all solids here - it looks like it's getting confused with handling the edges that are overlapping right on top of each other, normally that's more of an issue when the pieces are maybe slightly skimming each other a little bit and not as much of a problem when they are very exactly on top of each other but it looks like in your case here there's some bug where it's still getting confused about how to deal with the edges and seams that are stacked on top of one another.

If you rotate the spheres so that the "seam edge" of the sphere is inside the cylinder rather than resting right on the cylinder's own edge, it will behave better in this case, see attached example.


I would have thought though that your initial example should work since it looks like things are meeting very precisely, there is probably some kind of bug involved there but probably not an easy one to fix without potential side effects.

- Michael