Mobs glitch through walls (Multiplayer)

I have made a video which nearly explaines itself.
I have a mobfarm with a zombiespawner and a water/sign elevator for the mobs and on places where the zombies only have 1 block wide room they tend to glitch through walls AND i can kill them. On the video you can only see the glitching sadly i couldnt record an outbreak which happens sometimes!
Ive uploaded 2 videos:
https://youtu.be/NOSr00UsvVY
https://youtu.be/EJu7qs318KA