Mobs spawning where they shouldn't be able to.

Zombies spawn on half slabs & grass paths & spawn stuck half way in the ground. When standing on a half slab or grass path the light level is zero. I went afk waiting for chickens to grow & when I got back (after a full day & night cycle) half of my villagers were gone & zombie flesh was everywhere. Another time I went afk in my base & came back to 4 zombies in my house. The light levels in my house are 10 - up & light levels in the village are 9- up. I have torches everywhere to keep the place well lit but zombies are spawning anyways.