Welcome to the forum! No, it is not normal. There are a few possibilities. Your gas bottle could be nearly empty...not enough liquid left to supply the furnace but enough to get it to light. It could also be a faulty thermocouple. Could also be the excess flow check valve in the propane system is not resetting properly. Try turning the tank valve off and then opening s l o w l y a few light taps with a small hammer can also help reset a sticky valve. You might also have a faulty regulator at the tank that won't allow proper line pressure to the furnace.
Do any of the other LP appliances show unusual symptoms? Water heater work ok on propane?
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.