Comment 6 for bug 1898590

Turns out this seems to be a never ending story and you might have found a comeback of that issue for your particular configuration as you say this worked on 18.04 but fails on 20.04.

This goes way back
https://bugzilla.mindrot.org/show_bug.cgi?id=1455
Or half way back
https://trac.macports.org/ticket/49007
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=618863
https://bugzilla.mindrot.org/show_bug.cgi?id=2119

Other more recent similar issues were around "options edns0" being required to be set for this to work now:
https://github.com/NixOS/nixpkgs/issues/12470
https://exanames.typepad.com/blog/2009/06/one-more-thing-to-do-with-dnssec-ssh.html
https://bugzilla.redhat.com/show_bug.cgi?id=1630180
https://bugzilla.redhat.com/show_bug.cgi?id=1878166
Note: that option was the default for /etc/resolv.conf on Bionic/Focal for me.

Various working setups seem to have been affected by 7.5
https://lists.mindrot.org/pipermail/openssh-bugs/2017-April/017631.html
https://lists.mindrot.org/pipermail/openssh-unix-dev/2018-January/036600.html
https://bugzilla.mindrot.org/show_bug.cgi?id=2708

But Bionic -> Focal is openssh version 7.6 -> 8.3

Multiple of the above and some other references refer to requiring ldns support.
That clearly is in openssh since ~v6 but we don't enable it at build time
                   libldns support: no
Is that required and is it now more required than before - I don't know :-/

Sorry, all that I could provide so far was a collection of a (disturbing) history of that feature.