The Bug:

Some nether fortress platforms that generate through terrain don't have proper roofs.

See reasoning in MC-216221.

Here is an example:

Seed: -5497747566889979774
Coordinates: /execute in minecraft:the_nether run tp @s 106445.12 73.00 -1529.48 69.09 5.58

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 platform.
3. Take note as to whether or not some nether fortress platforms that generate through terrain have proper roofs.

Observed Behavior:

Some nether fortress platforms that generate through terrain don't have proper roofs.

Expected Behavior:

Some nether fortress platforms that generate through terrain would have proper roofs.