This is a very strange bug, very dodgy.
When exporting certain Unions or MeshParts, the objects that get exported are always a bit dodgy. Here’s an example.
The original object in-game:
The exported object in blender:
There’s not much more to this. It’s very inconsistent.
FOLLOWUP: It seems I have to select each part type separately, and this is by ClassName, not the root type. I had to individually select my Parts, export, individually select my UnionOperations and export, and individually select my MeshParts and export.