The exact replication steps aren't clear to me yet, but in some cases I find a couple of odd behaviours with building orbital units: a) In some cases the build bar stops progressing regardless of how long you keep building for. It seems to happen when you stop building and then resume, or when you add new builders to it. b) A minor cosmetic issue is that the orbital builders often don't point to the thing they are building. They'll just keep pointing wherever they were pointing, with a tiny bit of green stuff coming out of the front. c) I think I've seen construction bots sending 'nano particles' up to help build orbital.
Known bugs: PA#2959 It's was here since orbital engineers appear in game. Task: PA#2961 Not completely sure if this bug or feature, but terrestrial engineers can actually assist orbital construction. So you can't assist/resume orbital construction with orbital fabbers, but can do that with land/air.
