The green geometry on the left was the starting geometry. It was obtained from a sphere trimmed with a rectangle.
This geometry was exported to IGES format. Then, reimporting it into Rhino, gives a geometry different from the starting one. It is the red geometry, on the right.
Hi Pascal, I read the comments on the jetbrains.
I suspect it is not real an issue related to the import from IGES.
It sounds very related to the Trim Loops of revolution surfaces, before exporting.
This was a problem that I had (and still have) on the trim curves of trimmed brep faces on revolution surfaces:
(Again, I don’t really know if this is of any help for you.
If not, I apologize and please do not consider this post.)