I can’t really help here as it would require me to have a deeper look into your instance which is out of scope I am afraid. What I can tell for sure is:
Zammad will detect an existing user that it has not yet synced (or seen) via LDAP and then “map” it to the ldap user if the email address is the same.
If I’d have to guess I’d say that something causes Zammad to see the user from a different source first which might become troublesome. A normal email from said user wouldn’t be such a case normally.
