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.
Attachments
Comments
-3690904906524765661
-387 68 -443
Seemingly is, fortresses replacing blocks with air but not removing the support-needing blocks above
I recon this was never really an issue before 1.16 due to changes in how the nether is handled.

Dupe of MC-231712
@unknown No. That ticket does NOT mention anything about fire generating in the air.
Fire is NOT a nether plant (although this looks like at first).
Fannic, it is the same cause in the code, fixing either one fixes the other too. THAT is why it's a duplicate. I already requested that that report be made more accurate/also mention fire.
It turns out you are wrong. Placing fire with setblock command causes it to immediately disappear, while placing crimson roots with setblock command does not cause this, therefore suggesting that fire uses different code for placement.
Setblock is a VERY different situation than generation. They are non-comparable.
Setblock updating blocks inconsistently is already a separate report.
It's because fire executes block tick every single frame, so there is no way they could generate like this.
Seed and coordinates?