I was in Hardcore mode, 1.13.2, with no fire, no lava, no fire tools anywhere near me, no mods, only Optifine.
I was looking in the opposite direction. I turned around, and this very block (marked) was just.. burning? I cannot imagine how that happened. It wasn't the wood next to it burning, it was fire on top of the empty space. Has anyone ever experienced this?
Yes, that is how fires start, air blocks near lava catch fire and light flammable blocks.
I'm pretty sure you've got a lava block under that obsidian somewhere close.
From the Wiki:
"Fire will spread over flammable surfaces. Fire can climb up walls, across floors and ceilings, and over small gaps. More precisely, a fire block can turn any air block that is adjacent to a flammable block into a fire block. This can happen at a distance of up to one block downwards, one block sideways (including diagonals), and four blocks upwards of the original fire block (not the block the fire is on/next to). Therefore if the player is using fire to build a fireplace, caution is needed. Blocks in the way will not prevent fire from igniting blocks above it—so even if the player protects their wooden roof with cobblestone between it and the fire, the fire will completely ignore that cobblestone.
Fire spreads from a still lava block similarly: any air block one above and up to one block sideways (including diagonals) or two above and two blocks sideways (including diagonals) that is adjacent to a flammable block may be turned into a fire block."