We had this issue and thought it was the map. Turns out in multiplayer if there are more than 500 zombies near the player it will start deleting massive amounts of them. This issue was fixed for me using this mod (no mo culling) https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3115293671 Note that this mod must be manually installed on the server.