auth-zone primary: hostname@853#dns-over-tls.example.com

Unbound
1

I'm using unbound 1.18.0 as included in OpenBSD 7.4 to provide recursive
dns, and configured it with auth-zone to speed up lookups and changes.
it works just fine with this config:

auth-zone:
    name: "example.network"
    primary: ns.example.org
    allow-notify: 2001:db8::53 # raw IP of ns.example.org
    allow-notify: ns.example.org
    fallback-enabled: yes
    for-downstream: no
    for-upstream: yes

Specifically, primary uses the hostname to lookup the correct IPs to do
the AXFR from. However, when I try to switch it to use DNS-over-TLS for
AXFR, it fails:

auth-zone:
    name: "example.network"
    primary: ns.example.org@853#ns.example.org
    allow-notify: 2001:db8::53 # raw IP of ns.example.org
    allow-notify: ns.example.org
    fallback-enabled: yes
    for-downstream: no
    for-upstream: yes

I get this message in syslog,
  unbound: [76439:0] error: example.network.: failed lookup, cannot probe to master ns.example.org@853#ns.example.org

But when I change primary to
  primary: 2001:db8::53@853#ns.example.org

everything works fine.

Is this intentional?