John in NM
I've seen that same effect on pilots running propane too - usually on the 3-5 year time scale. People often replace the entire valve assembly, but I've also heard of cleaning the pilot jet work rather than replacement.
Coincidentally, all these were also at higher elevations - over 6000'. Less oxygen generally means a dirtier flame so I wouldn't be much surprised if it was due to that over the propane.