After starting the server, I noticed that several blocks were on fire. It didn't bother me, but after > 1h of real time the blocks are still burning (even when it rains).
Then I started investigating this and discovered that the fire doesn't spread, even when manually set on blocks that should be burning.

I tested it without any plugins

doFireTick = true
spawn-protection = 0

Vanilla server - bug does not occur.