It's impossible it's happening "no matter the aircraft" because, as you can see from the log, the aircraft has set the number of passengers to be 20 as part of its custom GSX integration, and GSX properly respect this, because the custom airplane code always have priority over anything else in GSX, under the assumption the airplane KNOWS how many passengers it has.
So no, it cannot happen with every aircraft.
Maybe you are referring to multiple variations of the FsLabs Airbus ? Because, as far as I know, only FsLabs initialize the variable to 20, to be sure in no situation you'll ever see a small bus, because the maximum number of passengers carried by the small bus is 17.
If you want to have the number of passengers set by FsLabs matching your Simbrief plan, you must be sure you are using the FsLabs correctly, so it would set the passenger number according to Simbrief. GSX won't override what the airplane code has already set, even if you have a Simbrief flightplan loaded in GSX.