Steps to reproduce

1. Create a new world.
2. Make a nether portal and go in it.
3. Find a naturally generated fire in the nether.
4. Place wooden planks or leaves all around the fire.

Expected result

The planks and leaves catch fire.

Observed result

The planks and leaves do not catch fire.

Fires spawned in during world generation in the Nether do not spread fires.

Putting random tick on 100, go to the Nether Wastes biome and find a patch of fire.
Place a flammable block next to it.

Compare to a player-started fire.

It's a very minor issue, but it creates odd interactions