The autheticator_diagnostic field is not filled by any protocol modules that use the new user account management system. This means that only the CDC protocol would work as expected and for that particular protocol the show dbuser command is not at all useful. To remedy this problem and to correctly display the users, the service REST API resource should display the users now that they are actually stored there.
Even if the users are added to the REST API, the output of the command is not that easy to read as the data is displayed as a pretty-printed JSON array. The data should be formatted into a table similar to how the list command display data.