You are observing the correct public DNS name for a host's public IPv6 address, as provided by the public DNS server of your ISP that is managing the respective IPv6 address space.
This has been asked and explained on various occasions, e.g. Client name for IPv6 addresses is sometimes resolved by upstream DNS server? - #2 by Bucking_Horn or Reverse DNS lookups of client's IPs - #6 by Bucking_Horn