Wither Skeletons can only spawn on Nether Bricks

What version did Shulkercraft build it in? The farm might be version exclusive since spawn rates change occasionally. 

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.

Please make sure the debug screen is enabled in at least one snapshot. If you don't know how to do this, press F3, and then take the snapshot.

This looks like a duplicate of MCPE-16292; however, that is a bug report for a different edition of minecraft. Does that still mean it is a duplicate?

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.

It might not be a duplicate to MC-237986, because wither skeletons still apparently spawn if the farm is removed. Also, the snapshots were taken in a hardcore world if that has anything to do with it. Can anyone confirm that this is an issue?

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.

This is not MC-237986 because Nether fortresses are not affected by that bug (I tested it).

Still broken in 21w42a

I tested my world (generated in 1.16.x and updated to 1.17.1) in the new 21w42a snapshot, and I too was not getting any spawns on my wither skeleton farm.
I generated a new world with the same seed and found out that the nether fortress doesn't generate at all, so that is why there were no wither skeletons spawning (any fortress mob obviously). The terrain was exactly the same (same blocks, same biomes), just the structures appear to spawn on different locations, breaking the farms.