synaptic can't download changelog known to apt-get

[Dirk F]

Package: synaptic
Version: 0.83+peppermint (and others)

After clicking 'Get Changelog' button in 'Description' tab of package
Properties pane or pop-up, the following text appears in the 'Changelog'
pop-up under the heading "Complete changelog of the latest version":

"This change is not coming from a source that supports changelogs.

Failed to fetch the changelog for coreutils
URI was:
http://gb.archive.ubuntu.com/pool/main/c/coreutils/coreutils_8.25-2ubuntu3~16.04_i386.changelog"

(In passing, the error message should be translated into English:
"This change comes from a source that does not support changelogs."
Or even more accurately
"Cannot retrieve changelog: either this change comes from a source that does not support changelogs or Synaptic does not support changelogs from this source.")

Whereas:

$ uname -a
Linux xxx 4.4.0-138-generic #164-Ubuntu SMP Wed Oct 3 15:02:00 UTC 2018
i686 i686 i686 GNU/Linux
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: LXLE Eclectica 16.04 32bit (16.04.5 LTS)
Release: 16.04
Codename: xenial
$ apt-get --version
apt 1.2.29 (i386)
Supported modules:
*Ver: Standard .deb
*Pkg: Debian dpkg interface (Priority 30)
 Pkg: Debian APT solver interface (Priority -1000)
 S.L: 'deb' Debian binary tree
 S.L: 'deb-src' Debian source tree
 Idx: Debian Source Index
 Idx: Debian Package Index
 Idx: Debian Translation Index
 Idx: Debian dpkg status file
 Idx: Debian deb file
 Idx: Debian dsc file
 Idx: Debian control file
 Idx: EDSP scenario file
$ apt-get changelog coreutils
Get:1 http://changelogs.ubuntu.com coreutils 8.25-2ubuntu3~16.04
Changelog [56.2 kB]
Fetched 56.2 kB in 0s (259 kB/s)
...

Surely this ought to work in the same way in synaptic?

synaptic would be able to find the changelog if its
RPackage::getCandidateOriginSiteUrls() returned a vector with 0th
component "changelogs.ubuntu.com", but it seems to assume that the
changelog domain is always that of the package file. One might suppose
that this method was introduced for generality but not fully
implemented, given that for origin!="Debian" it seems to be a no-op (and
also see line common/rpackage.cc#L961).

Also, the apt.conf manual page description of accessing changelogs under
Acquire::Changelogs::URI, which could certainly be a lot clearer,
doesn't seem to be respected and and in particular the settings
Acquire::Changelogs::URI::Origin::ORIGIN and/or
Acquire::Changelogs::URI::Override::Origin::ORIGIN appear to be ignored. See also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=902168.

In case the package maintainers produce an update that's really a new
program (Firefox ahem) or otherwise broken, the ability to view the
changelog is quite critical and ought to be properly supported in synaptic.