i was in a pillager outpost and as soon as i poured a lava bucket inside a 2x2 stone brick area, i started burning from a 1 block gap between me and the lava, then the whole outpost got affected and was out of control so i tried to turn fire spread off. After that i got out and when i reconnected (solo survival), in some places, fire was invisible but i could hear it and get damaged when standing on certain spots. The only way to find the fire was to place a block in the area where that fire was supposed to be on, and then the fire would magically appear. So in other news, fire could be in the air or at the side of a block.

In the pictures below i'm showing how there's no fire but when i take 1 more step i get burned, and when i place the dirt block below the fire, it becomes visible again.