Is this happening with the FBW A320 only ? Usually, it means:
- you must call the refueling service with a fuel quantity lower than the one planned.
- when the truck arrives, you should use the airplane own EFB to load fuel
- if the quel quantity is increased, GSX will detect that and will stop the message.
So, according to your report that fueling is progressing, it seems there must be some kind of automation going on that is interfering with what normally always work if you set the new fuel quantity manually.