Passengers arrive in crew Bus

[virtuali]

No, it means this occurs at any PAX number, low or high (for an A320) doesn't matter.

The you should really try to provide complete reproduction steps, including the precise, exact, order of all the operations you do, both in the aircraft and GSX, especially anything that might have something to do with payload.

[Brokenbroccoli]

Okay I try:
-Set valid flightplan using Simbrief
-spawn at a gate without jetway
-connect stairs via GSX
-load flightplan into Fenix EFB
-call fuel truck
-when fuel truck is ready load the aircraft using Fenix EFB
-as soon as refueling is completed, request boarding
-select "nobody"
->PAX arrive most of the time in the crew bus

and same for deboarding
settings:
[gsx]
followme_speed_knots = 13
slowdown_speed_knots = 7.5
straight_pushback_meters = 100.0
refuel_always_progressive = 0
detect_custom_refuel = 1
estimate_passengers = 1
assistance_services_auto_mode = 0
auto_mode_ignore_doors = 1
enable_game_cameras = 1
enable_editor_cameras = 1
enable_airport_walkers = 1
good_engine_start = 1
ignore_wingspan = 0
request_pushback = 0
multiple_trips = 0
ui_verbosity = 2
ui_volume = 10
marshaller_distance_readout = 0
deboarding_time_min = 15.0
deboarding_time_factor = 0.1
boarding_time_min = 15.0
boarding_time_factor = 0.2
refueling_time_min = 15.0
refueling_time_acceleration = 1.0
waiting_msg_delay = 10.0
pax_density = 3
show_vehicle_editor_help = 0
parking_editor_width = 1274
parking_editor_height = 712
parking_editor_sash = 226
followme_disable_flee = 0
creator_nickname =
gsx_aural_cues = 0

(I deleted "simbrief_username=" but I use it)

[virtuali]

Okay I try:
-Set valid flightplan using Simbrief
-spawn at a gate without jetway
-connect stairs via GSX
-load flightplan into Fenix EFB
-call fuel truck
-when fuel truck is ready load the aircraft using Fenix EFB
-as soon as refueling is completed, request boarding
-select "nobody"
->PAX arrive most of the time in the crew bus

I followed this as closely as possibly, my Simbrief plan was for 137 passengers and, according to your setting of "multiple trips" disabled, they arrived all together in a single large bus.

I don't see anything wrong in your setting, the only one you should probably disable is the detect_custom_refuel option, which automatically calls the Fuel truck as soon the airplane increases its fuel quantity, but it's only useful when the airplane has a simulation of a turnaround procedure and would start refueling itself after a while, but as far as I know, no MSFS airplane requires it, it was mainly used for the PMDG 777 in P3D.