Are the two areas connected by service roads? Currently the pathing system is unable to determine which of multiple separate road networks the fuel trucks should enter to get to the intended destination.
I raised this issue a couple months ago and by the sounds of it, they’re working on it: Fuel supply truck issues - #5 by Bobe
In the meantime, just make sure that if you have multiple vehicle checkpoints that everything is connected by service roads. It does make things a bit awkward in terms of your airport layout, but hopefully it’s just temporary.