Add-DbaAgDatabase: testing prerequisites for joining database to Availability Group failed

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I use Add-DbaAgDatabase as part of a script that moves databases between servers with AGs. All replicas are configured for automatic seeding.
Add-DbaAgDatabase fails with below error, whereas SQL statement "alter availability group ag_name add database db_name " works perfectly fine on the same server.

Error:
Testing prerequisites for joining database db_name to Availability Group ag_name failed. | All replicas are configured to use automatic seeding, but the database db_name was never backed up. Please backup the database or use manual seeding.