That's not a problem or something that should be fixed, and GSX has always been like this: the vehicles used for boarding are not supposed to be the same that are already parked on the stand, because we assume they are bringing the luggage from some other part of the terminal, to the airplane for boarding.
The ones that start parked are not supposed to leave: if you are starting this flight with boarding, they are just there doing nothing except decorate the parking stand, and that's how it's supposed to be. If, instead, you are landing, the deboarding will be performed by them, which will then leave (to carry luggage to another part of the terminal, such as the baggage claim area), and a subsequent boarding will be done by other two *different* vehicles.
Basically, the boarding vehicles are always coming from away and will leave again when they are done. The deboarding vehicles are spawned on the parking, and will leave ONLY when used to do deboarding: they will never perform boarding.
