With the 20w19a snapshot we see MC-173115 fixed which I agree with. However I think there is an inconsistency we might want to consider. I think the reporter of MC-173115 is correct in saying that generally speaking mobs should not spawn on/in blocks that they take damage from. For example, a zombified piglin can spawn on magma blocks because it doesn't take damage from it but a zombie cannot. I think this should extend to wither skeletons and wither roses for the same reason. They do not take damage from them and so should be able to spawn as before within them.

This change would bring consistency to the wither rose while not altering balance of most farms too much. Wither skeleton farms already use this mechanic to kill off other mobs quickly so these farms may receive a slight boost in rates I don't think it would make them over powered.