After upgrading from Spring Boot 3.0 to 3.1 today, MongoDB authentication via the autoconfiguration is broken.

If you configure both a database to use, and a separate database to authenticate against, the authentication database is ignored, and authentication is attempted against the to-be-used database.

In versions past, and the intended behavior is, that specifying the authentication database allows authenticating as the user to that DB and then switching over to the configured database for normal operations.

Perhaps the PropertiesMongoConnectionDetails should append the authSource option to the connection string if authenticationDatabse is present?