Database users are not transferred during migration.
The affected database users were created manually and without a database associated with them.
Plesk Migrator did not transfer the manually created database users because Plesk database contains no information about them. Plesk Migrator transfers only objects that were created via Plesk interface: service plans, subscriptions with all associated domains, and websites with content (such as websites, mail, databases, database users).
Manually create the same users on the target server.