The world has always been on the latest snapshot, but for the last couple the nether has been incredibly broken, so these are completely new chunks that have only been generated in this version.
The weird thing is Wither skeletons spawned fine without the spawn proofing or farm, just when I made the farm they refuse to spawn.
I've done some experimenting, and if I remove the buttons on other parts of the fortress, wither skeletons have no issue spawning. This includes if I remove the carpets on the outside of the farm.
My bad, this is LogicalGeekBoys farm, his video says 1.16/1.17 so it must be a new bug in the snapshot. I even tried replacing the spawning platform with dirt/wither roses and they still wouldnt spawn. No issue spawning a lot of piglins and zombified piglins.
Three of us are having this issue on a multi-player server in 14w21b.
Still broken in 21w42a