Can't direct you to a specific document that says proper way to refill forklift propane tanks, but logic & safety says you are correct.
The logic part is that propane is heavier than air ands falls toward the ground. So if one is refilling the tank on board and LP escapes from an improper connection, a hot exhaust pipe (generally located below the tank) could possibly ignite it or the LP could migrate into the engine compartment where there are potential spark sources (open wires, alternators, etc.).
You will find that safety compliant companies that have an on site LPG refill area that is located inside of a locket fenced in area and the gate isn't wide enough to drive a forklift inside & the refill hose is long enough to reach outside. Most laws on LP are governed by state laws. For example, in Texas an LP storage tank for home heating, ceramic making, etc must be locate X number of feet away from the house, trailer, etc. In Georgia they can be located right next to the house/trailer (guess it's one away for to control the population growth)

The other advantage of removing the tank & filling on the ground is it good physical exercise helps maintain the six packs abs when you reinstall it.