Minecraft wiki entry for Nether_Fortress, associated text: In rare cases, a fortress may not generate any indoor rooms except one single lava room, forcing the player to go to another fortress (or housing unit bastion) to find their first nether wart. This usually occurs when a portion of the exterior section of the fortress blocks the exit to the interior section of the lava well room.
Sometimes, two or more fortresses can generate close to or even within one another, ultimately creating an even larger cumulative fortress.
Under the lava well, there is an opening out of the fortress, with a single block of nether bricks from which the lava spreads out.
For 1.16 Nether biomes, the soul sand valley bone blocks and basalt pillars (down to within three blocks as-usual of a walkway) can replace more nether bricks. Less-often, crimson forest and warped forest huge fungi, nether wart blocks and warped wart blocks and crimson stems and warped stems can replace nether brick, most visibly along walls.
Nether fortresses, together with swamp huts, ocean monuments and pillager outposts, are the only four generated structures in the game that have special spawning rules within their bounding boxes.
Dungeons are not included, since the spawning condition is dictated by the spawner rather than the dungeon structure.