In MSFS 2020:
- All known reasons and all known possible solutions of passengers disappearing when they seat, are covered at Page 98 of the manual
In MSFS 2024:
- MSFS 2024 native airplanes are handled in a completely different (way simpler and reliable) way, but if you don't have the SU3 Beta, there are LOD bugs in SU2 that will make you *think* passengers disappeared, but they are just made invisible because of LOD loading bugs.
About the Fenix, specifically: if you start the Fenix installer and see there's a "micro-update", DO NOT UPDATE, it's not a real update, it's the Fenix installer that suggests there's an update every time a single file in the airplane changes. GSX must patch some files ( the model exterior .xml in MSFS 2020 or the attached_objects.cfg in 2024 ) to enable Seated Passengers, so this is detected as a fake "micro-update" but the Fenix installer so, if you perform the update, the Fenix installare will restore the file as it was before GSX patched it for Seated Passengers, so you'll lose Seated Passengers again. So, if you do one of these micro updates, you must enter AGAIN the FSDT Installer and re-enable Seated Passengers on the Fenix.