The FS Labs is supposed to set itself the number of passengers for GSX.
If the Van arrived first, it was because when the Van was created, the number of passengers set by the A320 was small enough for the Van, but then it apparently changed after GSX decided to use a Van. On the 2nd trip, a Bus arrived, because of the now higher passenger number.
So, from your report, it seems the A320 has set the passenger number too late, after GSX already selected the bus type, becoming correct only on the 2nd trip.
I suggest reporting this to FS Labs, perhaps asking them exactly HOW and WHEN you are supposed to set the the passenger number from the airplane FMC.