You set AlwaysOn Availability Group automatic seeding in an instance of Microsoft SQL Server 2016 or 2017. After you add the database to the Availability Group, you receive an error message that resembles the following in the instance that's holding the secondary replica:
Error 41169: An error occurred while attempting to access the availability group database with ID '%.*ls' in availability group '%.*ls'. The availability database is not found in the availability group configuration. Verify that the availability group name and availability database ID are correct, then retry the command.
This issue occurs because automatic seeding may randomly fail to initialize the availability database.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.