Some progress here. It appears that the Gutenberg addon for WPDM has a conflict with the REST API Addon. For reasons unknown to me, it registers its own endpoints in the same namespace as REST API like so:
register_rest_route( 'wpdm/v1', '/search-package', array(
'methods' ► 'GET',
'callback' ► array($this, 'searchPackages'),
) );
Now unfortunately, the routes are found, but I am unauthorized (401).