Unioning does not respect Collision Group membership—it gets ignored completely. Whenever I union parts, their Collision Group gets reset to the Default one, and then I have to assign the Union back to it’s intended Collision Group. This should not be the case. It should remember what group the parts were in, and the Union should be automatically put in that group as well.
This bug appears to happen everytime I union something, on any place, and should be quite easy to reproduce. You just need to create a new group in the Collision Group Editor, assign a part (or parts) to it, then union it, and you should notice the Union get reset to the Default collision group.
(In the gif below, I union a bunch of parts that are in the Environment collision group, and the resulted Union is put to the Default collision group. Separating the Union works as expected, putting parts in their original collision group.)
It looks to me like Union creation does not respect all necessary “properties” of the parts.
It’s kinda a similar thing when a Texture or Decal is applied to a part, and when you union it, those objects are gone in the resulted Union. But when separated, they are back inside that part. That’s expected, but Collision Groups should be treated on the same level as the parts’ color—it should be transferred to Union as well.