This actually also happened to me during our trip to UK in 2002.

At that time, the KISS valve design still had right-hand (?) turn spring inside which means manual operation of the valve is working to loosen the internal orifice. Eventually it got too loose and gas by-passed the o-ring spiking the loop - equivalent to a fail-open solenoid scenario.

It was quite entertaining afterward trying to determine the flow rate in liters/min using a beer pint and a stop watch as the calibration device with a bunch of Europeans who had no idea how to convert pints into liters...

Eventually we worked it out, tighten the internal parts and problem solved for the trip.

And Gordon changed the spring to left-hand turn and retrofited all previous owners. Now each time it is pressed, the spring is tightening the orifice.

But it is still recommended to clean and check the orifice and o-ring from time to time...