I am currently using 1.17.1 - 37.0.33. I built three different Wither Skeleton farms of different designs and could not get a single skeleton to spawn. So I wandered around half a dozen different Nether Fortresses and found not a single mob spawning, with the exception of Blazes but only from spawners. I never saw more that two Blaze spawn at the same time and anecdotally, the spawn rate seemed a bit slow. Mobs spawned in the biomes outside of the fortress' bounding box, but nothing spawned in the fortresses themselves. There were Striders and Magma Cubes in lava below the fortresses, but I am not certain if they spawned there or moved there. I came across a couple of Bastions in my travels and found Piglins and Piglin Brutes where they should be.

My first step in troubleshooting was to load the Vanilla 1.17.1 installation and almost immediately had Wither Skeletons falling into the kill chamber of my last farm build. I had Xaero's Minimap and Xaero's World Map mods loaded originally and removed them as I continued troubleshooting. When I loaded my game up with only the Forge 1.17.1-forge-37.0.33 installation, no more Wither Skeletons. So I never tried either of those mods out by themselves loaded with Forge as the problem seemed to be with Forge itself. Finally, according to this post by @Amun_Fortex they are seeing the same problem but also with Guardians. I have not yet bothered with an Ocean Monument in 1.17.1, but will find one and update if I am seeing the same issue.

Also, the http://vazkii.us/br101/ link in the Rules and EAQ is broken, returning a 404 error.

It looks as though they have a bug fix for this now:

Build: 1.17.1-37.0.40 - Thu Aug 19 05:44:39 GMT 2021 sizableshrimp: Fix entities not spawning in structures (#8020) This affects swamp huts, pillager outposts, ocean monuments, and nether fortresses