Unbound returns SERVFAIL instead of NXDOMAIN when PTR for an address that is dns64-synthesized from a non-existent IPv4 address is queried.

$ dig @::1 -x 1.2.3.4

; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.68.rc1.el6 <<>> @::1 -x 1.2.3.4
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 6088
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0

;; QUESTION SECTION:
;4.3.2.1.in-addr.arpa.		IN	PTR

;; AUTHORITY SECTION:
1.in-addr.arpa.		3587	IN	SOA	ns.apnic.net. read-txt-record-of-zone-first-dns-admin.apnic.net. 15763 7200 1800 604800 172800

;; Query time: 0 msec
;; SERVER: ::1#53(::1)
;; WHEN: Wed Oct 17 09:00:55 2018
;; MSG SIZE  rcvd: 126

$ dig @::1 -x 64:ff9b::0102:0304

; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.68.rc1.el6 <<>> @::1 -x 64:ff9b::0102:0304
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 62567
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;4.0.3.0.2.0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.b.9.f.f.4.6.0.0.ip6.arpa. IN PTR

;; Query time: 468 msec
;; SERVER: ::1#53(::1)
;; WHEN: Wed Oct 17 09:00:42 2018
;; MSG SIZE  rcvd: 90



I think the return value NXDOMAIN is lost somewhere between mesh.c and the modules but I've not yet tracked.  Note normal AAAA queries return NXDOMAIN as expected.