Funny thing!

 From:  Michael Gibson
4775.7 In reply to 4775.6 
Hi Felix - most likely the piece that looks weird was actually damaged in the original file as well, when you save a file MoI actually has to translate its internal object structures into the OpenNURBS structures which is what actually goes into the 3DM file.

Normally that doesn't really involve making any changes to the data, but it can in the case of objects with damaged or invalid trim curves on them - the 3DM file container will attempt to correct things but if the trim curve structure is really messed up it can potentially look a little different after you save the file and reload it.

Again, this appears to be caused by the base object in your boolean being made up of a self-intersecting surface that folds back over top of itself.

Having any additional object will not fix this - the surface that you see here is not going to work to use in a boolean:

Doing a boolean on that has created an object that is damaged and has a messed up trim curve structure, it will behave weird in several different ways - having it look like things are leaking out of it, and it can be possible for it to look a little different after saving and reloading.

Again, the main problem is that you can't really do booleans with objects that are made up of surfaces that fold back through themselves - surfaces that fold back through themselves will confuse the surface intersector and you'll get weird objects from that.

- Michael