@Ghost333 Thank you for that comment, I checked just now and yes, there was some kind of error in World Builder and the top half of the map was unusable for building. This usually happens when expanding the map boundaries. Apparently it happened some time ago when I abandoned the development of this map

Top-right starting position has mostly unbuildable terrain for some reason. Attempted first as Scrin and only had room to put down a reactor with nowhere to put down an extractor.
All other starting positions are fine. Second attempt started on the bottom-left and all terrain was fine. AI started in the top left was unable to build anything either and only made an expansion unit the whole match