According to RFC7050:
A node requiring information about the presence (or absence) of
NAT64, and one or more Pref64::/n used for protocol translation SHALL
send a DNS query for AAAA resource records of the Well-Known
IPv4-only Name (WKN) "ipv4only.arpa.".
This is what I had during my test:
root@MF286D:~# nslookup ipv4only.arpa.
Server: 127.0.0.1
Address: 127.0.0.1:53
Non-authoritative answer:
Name: ipv4only.arpa
Address: 192.0.0.171
Name: ipv4only.arpa
Address: 192.0.0.170
Non-authoritative answer:
Name: ipv4only.arpa
Address: 64:ff9b::c000:aa
Name: ipv4only.arpa
Address: 64:ff9b::c000:ab
In your case it was no such answer from DNS so you had to configure Pref64 manually.