The issue was caused by the wrong attributes maps in certain
circumstances after union intersections.
Namely issue might have happen when more than one iteration of
union was happening and it was caused by the fact that new faces
might be allocated on the same address as freed face from the
old mesh.
Didn't find a nicer fix for this apart from correcting the whole
attributes map after each union step.
We could try removing attributes for the meshes which are getting
deleted, but in asymptotic it's gonna to give exactly the same
complexity as the current approach.
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