In the OAuth2 agent profile, the help description for the allowed Grant Types field states:
"If no Grant Types (OAuth2 Flows) are configured then AUTHORIZATION_CODE flow would be permitted by default."
The docs contradict this:
"The set of Grant Types (OAuth2 Flows) that are permitted to be used by this client.
If no Grant Types (OAuth2 Flows) are configured nothing will be permitted."
The default value is 'Authorization Code'.
When the field is empty, authz code flow succeeds (help text is correct)