The Bug:

Nether fortress bridges that generate through terrain don't have proper roofs.

Ever since its introduction, nether fortresses have been able to generate "bridge" sections through solid terrain regions. This results in a weird "roof" of normal terrain blocks generating here instead, which doesn't make sense. Several other corridor paths have roofs made of nether bricks, which is what would have been expected to generate in such cases.

While this form of generation has been standard throughout the generation of fortresses it does not feel intentional and ends up breaking immersion somewhat. Several other structures would replace terrain blocks with their own blocks in cases like these, such as strongholds (pre-21w07a) which would replace non-air blocks with stone bricks and their variants, and mineshafts which replace air with planks. Therefore replacement of the blocks in question with nether bricks would be the expected result in these cases.

Here is an example:

Seed: -5497747566889979774
Coordinates: /execute in minecraft:the_nether run tp @s -710.07 67.00 -404.87 274.42 -0.80

Steps to Reproduce:

1. Generate a world with the seed provided above and teleport to the given coordinates.
2. Look closely at the nether fortress bridge.
3. Take note as to whether or not nether fortress bridges that generate through terrain have proper roofs.

Observed Behavior:

Nether fortress bridges that generate through terrain don't have proper roofs.

Expected Behavior:

Nether fortress bridges that generate through terrain would have proper roofs.