Wither Skeletons Not Spawning

Wither Skeletons don't spawn inside nether fortresses, even inside the bounding boxes. I wasn't getting this issue until 20w22a, but this could possibly be a bug that originated for 20w20a. I tried lowering the mob cap, afking high in the nether, and even placing down more nether bricks for the wither skeletons to spawn on, but to no result.