This bug happens when 2 players are available to unload and simultaneously load supplies into a vehicle.
Player one loads a truck and drives it to another base, then player 1 unloads supplies into a composition while holding the unload command. Player 2 will hold the load supplies command at the same time allowing for infinite rank progression.

I assume the solution could be to lock the possible experience gained to correlate to the supplies the truck can hold. Assuming the game recognizes when the truck has moved from one base to another. Would also be nice to see some rank progression just for transporting supplies similar to transporting players not just unloading them.