Something I have noticed is that zombies can suddenly just despawn mid-combat.
This happens often after you have been fighting very large hordes (100+ zombies) and I have a theory what might be the cause.
Since hordes have a location on the map and a circle around it where individual zombies will spawn, I suspect that when fighting a horde at the edge of this circle you might end up with the horde suddenly moving outside the spawning radius, causing all the spawned zombies from it to vanish.