Understood. It wasn't the Fenix it was the PMDG 777
We found PMDG has added some GSX integration without advertising it, and it surely sets the passengers numbers, but we don't know exactly how, we just found traces in the GSX log the airplane is setting the passengers number, and found confirmation in the 777 main .WASM file some GSX-specific variables are there, but I can't possibly say how and when they are set.
Should I need to restart the script engine between departure and arrival or is this part of the bug you noted? I don't have the option to turn off the script engine during cruise ticked
Please don't confuse the disabling of the GSX functions in cruise with a GSX restart. When you are in cruise and the option to disable GSX in cruise is Enabled, GSX will NOT restart, it will continue running, but it will stop querying the Navdata for nearby airports, to minimize the chance a bug in the Navdata API or a bug in the data itself might possibly cause a crash or a freeze of GSX, but even in this disabled state, GSX is still running.
A restart is completely different, and it's either done by you manually from the Couatl traybar icon, or automatically by the auto-restarter .exe when it detects the Couatl engine stopped running, so it auto-restarts it.
Not that this matters with your problem, since as I've said, the passenger number is NOT reset during a Restart, for the precise reason it won't be lost from the original flightplan.
So, the only possible reason why GSX "forgets" the passenger number, is because the airplane has reset it to a different value, which is a confirmed bug in the Fenix, not sure about the PMDG 777, other than being sure the airplane does set it a certain times, but I can't confirm it when.