Hi Sebastian, I'm pretty sure that those filled in holes in the Alibre import are a display mesh generation problem and not actually bad geometry. I also tested importing a 3DM of just the one single face with the numerous holes in it like this:



If you spin it around and look at this single surface from the backside, you can see all the trim curves for the holes are actually there but some bad triangles have been created between some of the holes:




That's really looking like a triangulation issue - probably the base geometry itself like the actual surface and edges are ok here and could be used for operations that work directly off of the geometry.

This kind of situation with one single surface that has a lot of little interior holes in it tends to be somewhat difficult for triangulators to handle, so that's not entirely surprising to see some problems in that area. If you need Alibre's triangulator to handle this you may need to split it up so that it's made up of a few long strips of surfaces instead of one big surface that has so many little internal trim holes inside of it.

- Michael