To reproduce, light youself on fire then immediately put it out, and you won't catch on fire. Do the same to mobs, and they will catch on fire, regardless of how short they were in fire.
Can confirm. The player has to take damage from fire, at least 3 times in order to remain being damaged whilst stood outside of fire. This behavior is inconsistent with mobs.
Can confirm. The player has to take damage from fire, at least 3 times in order to remain being damaged whilst stood outside of fire. This behavior is inconsistent with mobs.