My dedicated server has been running this same world for a little over two weeks now.
Earlier today some players noticed that they weren't seeing any zombies anymore, So I logged in and looked at the server statistics and sure enough, only five zombies exist in the world.
If I spawn a zombie and then teleport away and teleport back, that zombie de-spawns.
If I restart the server, zombies will respawn all at once regardless of where they were prior, and then after 30 or so minutes, they all vanish again.
I'm not sure if this is because we altered the Zombie Respawn rate mid-game, or what... but it's pretty game breaking right now.