Facing the same issue but it seems to happen randomly with no fixed onset of conditions.
I've tried it with the PMDG 777, CS 757 and Majestic Q400.
Flying out of FlyTampa Vienna with a jetway stand, jetway docks, no passengers seen boarding. Same flight at destination T2G Doha and passengers deboard fine on a non-jetway stand.
Flying out of MK Studios Lisbon without a jetway stand, bus arrives, doors open, no passengers board. Same flight at destination AS Madeira Evolution and passengers deboard fine on a non-jetway stand.
Flying out of FlyTampa Athens without a jetway stand, bus arrives, doors open, passengers board. Same flight at destination ORBX Dubrovnik and passengers do not deboard on a non-jetway stand.
Then there are times when it would work perfectly at both departure and destination airports with the same aircraft above. I can't find a consistent fault so it's very difficult to reproduce, even for me, but there's definitely something that bugs out within GSX.