Bob Engelhardt writes:

I've been meaning to do the chemistry to show that there simply is not
enough energy from the propane in an empty tank to do any harm. But I
haven't gotten around to it (what else is new?).

Yeah, do that chemistry. You essentially have 5 gallons of steam at, what,
3000 deg F? Fair bit of pressure, that, and too quick for the overpressure
device to respond.