I think you might be incorrect on that one. I've had a HF adaptor for years now, and invert the larger tank when refilling the smaller ones.
IIRC the overfill protection valve is a float which rises to shut off the flow passage when the tank is overfilled with liquid while standing upright. When you invert the tank as when using the HF adaptor to refill a small tank the float rises towards the bottom of the tank, which leaves the valve passage open, if there's a decent amount of liquid in the larger tank.
Course I could be wrong about that, maybe the latest valves have a secondary mechanism to close off the valve passage when the tank is inverted?
One thing I always wondered about was overfilling a smaller tank so that it might vent (or G-d forbid burst?) if the surrounding temperature rose. I'm chicken about that kind of thing so I weighed a couple of just purchased tanks on the kitchen scale, wrote the weights on them and check them after refilling to make sure they're not above their marked weights.
Agreement/disagreement or clarification of any ofwhat I just wrote would be welcomed.
Jeff