Temps at high altitude tend to be a heluva lot colder than 31F. Frost on the propane line isn't usually a problem (but with butane it could be.) Don't blame the fuel- they use propane as heating fuel with above-ground tanks in places with design temps in the negative teens, and you BET the tank temps are well below 31F!
Careful about looking at pipes with infra-red- the emissivity of bare metals can be quite different from the IR thermometer's presumption. It'll read significantly on the low side for any shiny bare metal.