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 bircks. If you replace the platform with netherrack or stone, mobs stop spawning. This bug appear in 1.19.1 and 1.19.2, both vanilla and modded.

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