Any of the nether fortress mobs aren't spawning on any blocks except nether bricks. If you build any platform at nether fortress, mobs are spawning only on nether bricks. If you replace the platform with netherrack or stone, mobs stop spawning. Even with wither roses placed any wither skeleton won't spawn. This bug occurs since 1.19

Here is video evidence of bug in vanilla enviroment:
https://youtu.be/Cb6CPa3i4ec