I cannot answer, because that was not a personal tank. I could have opened it without a regulator, but I didn't think of that.One good question is, after the dive did the tank contain any air?
I would guess so.
To me the only possible way of suddenly "loosing" ~100bars of gas is something blocking the flow. Either something clogging the filter or the dip tube (if there was any) or something similar. Physically it is not possible to loose that much gas without noticing. We are talking about a lots of gas here...