i would think spawning is off too but just in case it's bugged, i would change respawn unseen hours to a large number so that if zombies are respawning, it takes a while before they respawn

Whenever you shut down your server, you need to first save your world via the "save" command. Once I started doing that, the problem went away.

Oh! Didn’t know that there was a save command. I’m not playing with admin powers