I assume you used a gate with a Jetway. If yes, the problem doesn't have anything to do with GSX, and it's likely the jetway has docked too low, with its roof not aligned to the airplane door threshold.
All waypoints in the jetway up to the point of contact with the door comes from MSFS own Navdata, so we don't have any choice other than follow these, since the jetway is dynamic and we can't predict exactly how it will move.
After passengers starts to follow GSX-defined waypoints, they get corrected so, the effect you are seeing, is passengers raising from the wrong waypoint obtained by the simulator, to the correct one aligned with the GSX-own door definition.
