Here's my tale of woe regarding fire.
I had a pit kiln embedded into cobblestone, 2 blocks up and 1 across I had a wooden tool rack. I didn't think about it when I placed it, although I know pit kiln fire does spread.
I couldn't figure out why my tools kept going missing, and what was happening was it would burn the rack but... that also destroys the tools at the same time!. I'd come back find an empty wall, think "Oh, I was going to put a tool rack there", place it, move tools over to it and a day later the process repeated itself.
It was only when I came to put up the tool rack the 3rd time, cursing my ADHD for thinking about doing it before and not doing it (although, obvs, I had..), and looking for tools to fill it that I couldn't find them. I took me a good 10 minutes of running around the base, searching every storage container, every nook and cranny, that I had my epiphany.
Moral of the story, even if you think it's far enough away, always move it further away.