I've tested a bit further - it does not make a difference if the third Stair is available or not! (meaning L4 is enabled / removed in Aircraft Customization for the QW787)
The Problem seems to be with Door L2!
When Deboard is finished (FSDT_GSX_DEBOARDING_STATE== 6) and I call Refuel, I can see the Toggle Requests from GSX for L1 and L4 (if avail/configured) and let the Doors close automatically. But there is no Trigger for L2! Even in the Menu there is no Text "waiting for ...".
Even when I now go ahead and close L2 manually and even say "Operate Stairs" to GSX - nothing will be moving. It is stuck, the State for FSDT_GSX_STAIRS will stay in 5 and FSDT_GSX_REFUELING_STATE will stay in 4 (no it is not that I not waited long enough). No Stair moving, no Truck coming. The only Difference with L4: the third Stair is indeed moved.
GSX clearly wants to have all Stairs removed (for whatever Reason), but does not "handle" L2 in that Case (not sending Trigger, not checking State). For Boarding or Deboarding (== 4 or 5) it does send Triggers for L2 and checks the State normally.
I've worked my way around now, with operating the Stairs as soon as Deboarding is finished. In that case GSX does send a Trigger for L2 just like normal. All Stairs are moving to their Park (State goes to 1) and calling the Refuel then is working normally.
When starting in a "Departure State" (GSX Ground-Handling active / Vehicles in Park-Position) and then calling the Stairs and then Refuel it is even worse: no Trigger is received, no Stairs moving, no Truck coming.
GSX in general does not want to have the Stairs in Place while Refueling. But I find it a bit unrealistic that no Stairs are connected to the Plane until it is boarded - Is the plane prepared for Flight by an invisible Hand? Do the Pilots, Ground-Personel or Engineers "beam" into the Plane? It would be nice to have at least the L1 Door connected/in Place!