host_name field does not match spec
Host name as found in a reverse DNS lookup of the relay's primary IP address. This field is updated at most once in 12 hours, unless the relay IP address changes. Omitted if the relay IP address was not looked up, if no lookup request was successful yet, or if no A record was found matching the PTR record.
There are many relays for which the host_name field contains the IP address, and there will be no A record for IP addresses, so the spec does not match what onionoo does? There are only 2 relays for which onionoo does not provide the host_name field.
tickets about the same field (but not the same issue): #18342 (moved)