Maybe the number of passengers (related to payload) changed for some reason in the exact moment just a second after GSX already decided which bus to use, that's the only thing that comes to my mind but, again, without any sort of details, it looks as if GSX commonly does this, which clearly is not the case.
Well, I think something IS messed up when the system allows 40+ passengers to exit the small van, even if something changed on the way (which it didn't in my case).
But I get that it's important for you to be able to reproduce the situation in which it occurs, so maybe this helps:
Note that this error doesn't appear every time, only in some cases - with the same workflow of flight preparation I always use.
1. Loading scenario, FSL A320, cold&dark.
2. Calling Catering (primarily to have stairs at the aircraft in the first place, otherwise how do I get up there, right?
)
3. Powering up (APU running).
4. Entering( only) fuel in the INIT page
5. Calling fuel truck
6. Entering passengers/payload via the MCDU options page (not the INIT page). I.e. pax 30/40/40, bags 1200 - 0 - 500 - 0
7. Calling for boarding (catering ist still working, didn't touch the INIT page)
8. Waiting. After catering is finished, Fuel truck arrives. Fuels correctly and leaves. Then Crew bus arrives. Two pilots exit, 4 FA's exit.
9. Crew bus leaves. Small van arrives (aha!)
10. About 90 people exit the small van.
11. Small van leaves. Bus arrives. The rest of the pax exit the bus and board the plane.
12. After "boarding complete" I continue with programming the FMC.
Maybe my procedure is flawed but as I said, sometimes it works (correct busses) sometime it doesn't (small vans).
I never fly the A320 below 50pax.
BR
Stu