Wither Skeleton Not Spawning

I am playing on the ps5 preview version and I have noticed that no wither skeletons are spawning. I have been to 4 nether fortresses and have created platforms for them to spawn on. I have removed all sources of light and brought the light level down and none have spawned. I have also put slabs down on all blocks around and that has not worked either. Thank you for your time and I hope this email finds you well.