Where does the propane go?

D.E.Bishop wrote:
In summary, the liquid propane in the tank becomes a vapor when released, it then becomes a flammable gas when it is further atomized when passing through the orifice in the nozzle and then oxygen is introduced in the burner tube and passes into the burner as a flammable gas.

Not quite.

Propane exists as both a liquid and gas in the tank or bottle. Since the valve is on the top, gas only is drawn off for use in the system. The pressure in the tank is the same for a given temperature whether the tank is full or nearly empty and can vary from about 100 to over 200 psi depending on temperature. The regulator drops the pressure to a constant 6 ounces per square inch. The gas is metered through the burner valve and mixed with air in the burner tube. There are no different properties of "static" vs "flowing" propane vapor.

Old-Biscuit wrote:
That is because LP regulator has opened to provide 0.4 psi in downstream system......but then closes back down until demand is required.
Upon demand the little bit of propane in system is not sufficient for ignition (or maintaining ignition ) until higher flow is established, which happens when reg senses demand and drop in line pressure.
Lighting off stove top burners helps establish flow and demand which places regulator fully in service vs static state when not being used.

The regulator definitely controls flow rate while maintaining constant pressure, but this narrative doesn't explain why a burner will light almost instantly if it has been used recently, but not if it has been a few days or more. The only reasonable explanation is that the low pressure gas in the manifold downstream of the regulator slowly leaks off over time. I guess someone would need to leave a manometer attached to the system and monitor it over time.