It looks to me like the flame got back to the orifice and started heating up the hose connection to the point of melting the hose itself. I've seen that happen a couple of times on our rig but we catch it because the heat on the burner falls off and we lose boil. The significantly different jet engine sounding noise is a giveaway as well. On our rig there is more distance between the orifice and the hose connection due to some additional safety valves so it takes quite a bit longer for the heat to transfer all the way to the hose.
This isn't really an issue with the regulator, but one of the hose and the connection to the burner. The stainless steel braid looks good, but if the thermoplastic hose gets warm it starts to ooze out and will develop a leak. This won't occur with a short term heat flash.