The bug

Because of structure generation, fire can sometimes generate without a supporting block.

To reproduce

Seed: -3690904906524765661
Coordinates: /execute in minecraft:the_nether run tp @s -386 67.00 -443.78 168 -18

To make sure it is not the same as nether plants use the setblock command in the air with fire and crimson roots:

/setblock ~ ~ ~ fire
/setblock ~ ~ ~ crimson_roots

Then, only the fire disappears instantly while crimson roots waits for the block next to it to update, suggesting that a different code is used for fire placement.