This has been explained several times on the forum already: if an airplane has a door that is lower than the minimum height a staircase or a vehicle can reach, the vehicle will not appear, assuming that airplane would probably have its own staircase.
In the sim.cfg of the vehicles supplied with GSX, it's possible to check such constraints, that match the physical extension that model can reach so, of course, even if you tweak of remove the constrain, the stair will appear, but won't be aligned to the door anyway, since it couldn't go that lower.
So, it's not a GSX bug, the behavior is entirely normal and by design, we might just have to add new smaller staircases models, that will be used on the smaller airplanes (which usually have their own staircases anyway)
