That's exactly how is supposed to work.
With the previous versions, with the option to show the default fuel page or not, you didn't "disabled" the fuel truck, you were only disabling the display of the default fuel page.
With the current version, that option controls if GSX should try to refuel the airplane itself, or if the refueling is done using another system, which can be provided by the aircraft itself, or might be a different program that changes the fuel.
So, if your reference to FSPassengers, was made to indicate you use this program to load fuel, the correct usage is to call the GSX fuel truck and WAIT until you see the message "Please use your airplane fuel system". Then, use whatever feature this program has to load fuel, so the fuel truck will not try to change the fuel by itself, and will act just as a passive animation, lasting as long as the external refueling lasts.
