Mobs Sink Into Block Underneath Them When Hit

When I hit a mob (only tested with enchanted diamond sword) it glitches through the block under it and if there's not a block under that block, it will continue to fall (hilariously in one case to it's death).

What I was expecting to happen: The mob to be slightly knocked back and stay an the same Y co-ord

How To Reproduce:
1. Find a mob
2. Hit it.
3. It will glitch through the block it's standing on