Mobs glitch through floor (upon being hit or chunk reload)

In an abandoned mineshaft, hit a zombie, he did the slight knockback jump, and fell through the one block floor. Additionally, I often have villagers glitching through the 1 block thick ground into the underground evil lab I have below the village on chunk reload. I never provided them with the door password, let alone it is out of their wander radius. Very rude of them to drop in unannounced.