When I try to summon Mobs for testing a Grinder Model, the Mobs simply fall through the floor instead of dying when they hit the bottom block.

What I expected to happen was...:
For Mobs to spawn, go into the tube, fall 30 Blocks onto a hopper, die, and drop materials

What actually happened was...:
Mobs Spawned, Successfully swam the water down into the pipe, fell 30 Blocks, hit the hopper, didn't die, and instantly fell through everything below all the way to the void.

Steps to Reproduce:
1. Use a Command Block to summon a Monster: IE; Creeper, Zombie, etc
2. Make sure to spawn them in water flowing into a pipe with a hopper at the bottom
3. Make the pipe about 30 blocks height. Then test the Command.

I attached a Slow Motion Capture of the witch falling straight through the ground.

Also sometimes this bug would randomly not occur if I re logged, but then start again if a re logged, its like a random Chance I guess?