I was originally using version 1.17.1 - 37.0.30 and noticed a problem with the Wither Skeleton not spawning in the Nether Fortress.
According to the Changelog, version 1.17.1-37.0.40 fixed the spawning in certain structures, so I updated to the latest version, 1.17.1-37.0.42.
However, I could not find Wither Skeleton in the existing world. So I regenerated the world with the latest version (No-MOD) and visited some Nether Fortress and found Wither Skeleton and Blaze on Soul Sand, Soul Soil, Crimson Nylium and Netherrack . However, I couldn't find Wither Skeleton on Nether Bricks. Therefore, I thought that none of the mobs spawned inside the Nether Fortress (on the Nether Bricks), so I visited the same Nether Fortress in Forge and Vanilla using the same seed and compared the results.
As a result, Wither Skeleton and blaze were found in the Vanilla Nether Fortress, while no mobs were found in the Forge Nether Fortress.
Each of the mobs were seen around Ocean Monument and Pillager outpost. Therefore, this may be a problem limited to the Nether Fortress. I would appreciate it if you could address this issue.