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.