Sometimes for stopover flights, airport security will check the boarding passes of passengers leaving the aircraft, to ensure that everyone is going to their correct destination. If you don't have BKK as your final destination on your boarding pass, they might not let you "arrive" in Bangkok. I'm not sure what the procedure is for this flight, ie whether everyone gets off the plane, or only those arriving in Bangkok. Just raising the possibility that you might need a correct boarding pass to arrive in Bangkok.