1.17.10 with experimental lush cave features turned on. Wither skeletons don't seem to spawn on wither roses. How to reproduce: create a typical wither skeleton farm using one spawning spot from the nether fortress and a trident killer. See the farm killing blazes, wither skeletons and some other mobs (depending on the biome). See them spawn on a netherack block too. Now place a wither rose on the netherack block mobs spawn on and see nothing spawn anymore.