Hey People that use Zombies, today i will release a pre fix for the issue you might have that some entities are still stuck.
What has been added?

When an entity died or despawned for no reason, it will be counted down from the left zombies like you would kill an entity, this should fix the issue if you have despawn settings at spigot setuped. What has been also added is an toggleable LivingTime for Entites in the main config.yml.
This new setting will automatically remove the entities after the seconds are over.

This is the default Configuration:

(The Value is an number in seconds)​