I ensure their new parking positions are; on the same side as the cargo doors; far enough away for manoeuvring; and normally pointing towards their respective door.
That seems to be enough but, of course, without looking at an actual example, is difficult to understand what the issue might be.
Even if it sounds counter-intuitive, such issues happens because GSX is not as dumb as just "following a path" but, in order to allow a realistic maneuvering with smooth curves, it tries to FIT A BEZIER CURVE along the waypoints a vehicle will use, with some constraints like having the vehicle ending up with the proper heading and aligned to the related airplane door.
When the curve cannot fit all the constraints, you'll see such issues like vehicles doing a 180 or passing over the other side.
Please provide me with an example, possibly using a default scenery with a default airplane, so I could try it and tell you how to improve the starting position of the vehicles.
Note that, the airplane parked position matters too so, you'll probably want to be sure the GSX parking position is set correctly as well.