as soon as I hit the load fuel button, the fuel starts to flow long before the truck arrives
That's because you haven't followed the GSX instructions that, when an airplane use a custom fuel system, requires you to load the fuel using the plane own system AFTER you see the message "Please use the refueling system of your airplane to refuel"
A dismiss or disconnect or manual fueling completed option would be great.
The "Reset position" or "Restart Couatl". They do the same thing.