No the “admin user” does not exist, we have personal accounts and one administrative user called “admin”. This fullname in the useradministration is “admin admin” and not “admin user”
Right.
I’d bet that the user is actually User-ID 1 (system user).
Usually Zammad would display - - in the history unless you rename the firstname and lastname.
In your case the chosen wording is more than confusing I guess.
The system user appearing in the history is technically correct and caused by e.g. triggers that are sent. Trigger mails are always fired by the system.
Your history doesn’t show the assignment so I guess this is just a wording thing. The assignment is shown within the owner field.