Hey
i know that there is a thread on this but it was for the PMDG 737 and the manual page 98 mentioned in that thread as the fix did not help.
But otherwise the issue is similar as that the passengers dissappear right as they are at the seat that they are supposed to sit in, Object LOD is set to 200, cockpit door open, ran the GSX config tool multiple times, checked for updates and errors/missing files in the FSDT tool, tried different liveries on the MAX including the default ones but still the seated passengers only show in the external view and no pink textures.
i did however find a gsx.cfg file in the ifly-aircraft-737max8\SimObjects\Airplanes\iFly 737-MAX8 in the community folder where these 3 look like this:
paxseatspos = []
crewseatspos = []
corridorsoffset = []