The resource-id is explicitly not set in the response because of performance reasons.
Due to the fact that the PDP can anyway only handle a single resource per request the response can be assigned anyway correctly.
For setting the ResourceId in the response, all attributes in the request-resource must be walked through to find the id. Then this id must be planted into the response.
For users who need that resource-id in the response the ResponseMarshaller could be extended so that it is possible to set a configuration option so that the resource-id is available. Per default this option would be turned off.