Zombies not spawning in cave/under ceiling

Zombies aren't spawning under ceilings since the update. To reproduce: create a flat world in creative, create a 100x100 ceiling at y=100 with the /fill commands, walk/fly around (optionally using nighvision potion). Under the ceiling, zombies/zombie villagers aren't spawning, even after multiple passes, only creepers & skeletons & spiders. Right outside of the area with the ceiling, they are spawning (even packs)