The bug
If the gamerule doFireTick is set to false, existing fire blocks will continue to not tick even if that gamerule is set back to true, and as such don't go out or spread to other blocks.
To reproduce
/gamerule doFireTick falsePlace fire
/gamerule doFireTick trueWait
→ ❌ The fire never goes out or spreads to other blocks
Wait this is considered a bug? This has been the behavior for as long as that gamerule has existed (back to 2013 or 2012).