DNS64: PTR synthesis yields SERVFAIL for some cache contents

Summary

When cache is cold, PTR synthesis of DNS64 module works well. When cache gets populated by quering without DNS64 synthesis on, PTR synthesis stops working and SERVFAIL is returned instead.

Steps to reproduce

# cat /etc/knot-resolver/kresd.conf 
-- SPDX-License-Identifier: CC0-1.0
-- vim:syntax=lua:set ts=4 sw=4:
-- Refer to manual: https://knot-resolver.readthedocs.org/en/stable/

-- Network interface configuration
net.listen('127.0.0.1', 53, { kind = 'dns' })
net.listen('127.0.0.1', 853, { kind = 'tls' })
--net.listen('127.0.0.1', 443, { kind = 'doh2' })
net.listen('::1', 53, { kind = 'dns', freebind = true })
net.listen('::1', 853, { kind = 'tls', freebind = true })
--net.listen('::1', 443, { kind = 'doh2' })

-- Load useful modules
modules = {
        'hints > iterate',  -- Allow loading /etc/hosts or custom root hints
        'stats',            -- Track internal statistics
        'predict',          -- Prefetch expiring/frequent records
        'dns64',
        'view',
}

-- Disable DNS64 for IPv4
view:addr('0.0.0.0/0', policy.all(policy.FLAGS('DNS64_DISABLE')))

-- Cache size
cache.size = 100 * MB

First query over IPv6 works as expected:

# kdig @::1 -x 64:ff9b::101:101 +noall +answer 
1.0.1.0.1.0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.b.9.f.f.4.6.0.0.ip6.arpa. 60    IN      CNAME 1.1.1.1.in-addr.arpa.
1.1.1.1.in-addr.arpa.   1265    IN      PTR     one.one.one.one.

Query over IPv4, where DNS64 is disabled, also works properly with NXDOMAIN:

# kdig @127.0.0.1 -x 64:ff9b::101:101  
;; ->>HEADER<<- opcode: QUERY; status: NXDOMAIN; id: 41713
;; Flags: qr rd ra; QUERY: 1; ANSWER: 0; AUTHORITY: 1; ADDITIONAL: 0

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

;; AUTHORITY SECTION:
ip6.arpa.               3600    IN      SOA     b.ip6-servers.arpa. nstld.iana.org. 2021111921 1800 900 604800 3600

After this query, PTR synthesis does not work anymore and yields SERVFAIL:

# kdig @::1 -x 64:ff9b::101:101  
;; ->>HEADER<<- opcode: QUERY; status: SERVFAIL; id: 25807
;; Flags: qr rd ra; QUERY: 1; ANSWER: 0; AUTHORITY: 0; ADDITIONAL: 0

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

Clearing the cache restores correct behavior for a while.