Why does an incident thread entry show "Unknown author"?
Incidents thread title bar, Deleted staff accounts
Also applies to deleted Contact records
Oracle Service Cloud, All versions
Thread entries can show "Unknown author" when the staff account that made the entries has been deleted. This association comes from the account ID at the database level. When the account is deleted, the account ID is also removed. The application can no longer display the staff member's name because the association no longer exists.
For this reason, it is recommended to disable staff accounts rather than delete them. For more information on this, see the following answers: