Steps to reproduce

1 Create custom admin user with standart admin role(for example admin1)
2 Create Approve workflow and assign approving of content to this "admin1" user.
3 Assign this workflow to before publish trigger.
4 Create an object(by edit user) which will wait an approval from "admin1" user.
5 Login admin user and remove "admin1" user.
6 Create another admin castom user and you won't be able to see your published user in user section, because of workflow issue.

After admin custom user removal all created objects will wait approval from non-existent user("admin1").

Try to edit this workflow and then click Cancel button, as result you won't be able to see this workflow from admin interface, only in database.