I found exact same issue with my powered liftgate, which only close properly if the door or window is opened even just a little bit. I guess it has something to do with the pressure inside the car as you mentioned. If we power-close the rear-hatch when doors/windows are all closed, then the "ajar" shows up on the dash. Also manual-close work fine each time.
Do you still remember how you got this fixed?
Thank you!