After some trouble shooting and testing we seem to have fixed this. It was related to the zombies movement speed (the command for it specifically) we've used a different command for movement. Let us know if it bugs out again.
See:
https://www.invexgaming.com.au/showthread.php?tid=1578
Moving to solved.
See:
https://www.invexgaming.com.au/showthread.php?tid=1578
Moving to solved.