It seems that the propane filler is filling the tanks until the OPD Valve shuts off. That is an illegal way of filling propane. The OPD is there only as a secondary measure and the cylinders are ONLY allowed to be filled either by weight or until the dip tube / bleeder screw starts spitting. The OPD function would tell the filler when to stop but they would still be able to bypass that by putting in more propane (a common mistake). Take your car for example, when filling it with gas the pump will shut off when it's time to stop but you could still technically bypass that and put in a bit more.