EDIT September 14th, 2020: Zombie kills in the next Zombie Horde Special Ops will now be counted correctly. We have sent a small compensation to all affected players. Once again, please accept our apologies for any frustrations caused by this.
There is currently an issue in the game with the amount of Zombies killed not counting towards the Zombie Chest correctly. We are investigating this issue here. Our apologies for any inconvenience caused by this.