I know there are other tickets for this, but this has been a game-breaking bug for months now according to other bug reports I've found. Removing the double landing gear wheels from my creations has fixed it for me, so from what I can tell, it's related to that and not "placing seats on subgrids" as some others have said. This bug makes multiplayer literally unplayable with most of my creations, please guys figure this out soon :(