I believe what you're seeing is normal behavior. If you ran that query against a TLD you'd get the NS servers of the TLD in the answer section but I don't believe it works that way when delegating subdomains.
I just tested in my environment and when I query an a-record in the delegated subdomain the proper NS records appear in the authority section but I do not get a response querying for NS records directly against a delegated subdomain.