raid mobs spawning on leaves and bottom slabs even though they have several full blocks to spawn on instead

I have a raid farm (JCplayz' design) but the mobs keep on spawning outside their designated area which has plenty of room. They shouldnt be able to spawn on the leaves or bottom slsbs that I added but they are. On top of that they are spawning inside the wall of the villager's rooms and killing it. Please fix this.