I tested it now, and I cannot replicate it, as you can see in the video, GSX is setting the planned fuel quantity in the right FMC at the right time, and the fuel truck counter starts as soon the quantity changes, see the video:
That is, if you use Simbrief, of course.
If you don't use Simbrief, so you are just inserting the quantity manually, it's up to you to be sure the quantity you are setting is HIGHER than the quantity you had before calling the GSX fuel truck, this is because the truck waits for a fuel increase to start.
That's why, when you use Simbrief and call GSX refueling, as part of the automation, GSX first set the fuel to 5%, call the Truck, and when the truck is in position, sets the final quantity according to the flightplan, so it will always an increase. But if you are not using Simbrief, it's up to you to do this.