End user experience:

 - Klick "restart and update" in GNOME Software
 - Confirm restart
 - System boots
 - After a few minutes, system boots again
 - Logging in and restarting GNOME softwrare, the same list of updates is
displayed again, with the same "Restart and Update" button
 - This can be repeated

journalctl -b -1 shows this:

> Jan 16 08:37:49 apollon pk-offline-update[1406]: status wait
> Jan 16 08:37:49 apollon pk-offline-update[1406]: status setup
> Jan 16 08:37:52 apollon pk-offline-update[1406]: status finished
> Jan 16 08:37:52 apollon PackageKit[1449]: update-packages transaction /1_ebebddca from uid 0 finished with failed after 2901ms
> Jan 16 08:37:52 apollon pk-offline-update[1406]: writing failed results
> Jan 16 08:37:52 apollon pk-offline-update[1406]: failed to update system: There is no update candidate for post-build-checks-84.88+git>
> Jan 16 08:37:52 apollon systemd[1]: systemd-rfkill.service: Succeeded.
> Jan 16 08:38:02 apollon pk-offline-update[1406]: rebooting
> Jan 16 08:38:02 apollon pk-offline-update[1406]: sent mode to plymouth 'shutdown'
> Jan 16 08:38:02 apollon pk-offline-update[1406]: sent msg to plymouth 'Rebooting after installing updates���'
> Jan 16 08:38:02 apollon systemd[1]: packagekit-offline-update.service: Succeeded.
> Jan 16 08:38:02 apollon systemd[1]: Stopped Update the operating system whilst offline.

So, pk-offline-update returns Success status even though it failed to do the
update. 

The actual update problem was due to a version "correction" in
post-build-checks that would cause the package to be downgraded:

> ----------------------------------------------------------------------------
> r95 | dimstar_suse | 2020-01-10 16:47:24 | 7568a213422b18c1c6714e41fb98c0b2 | 84.87+git20200110.2d02f07 | rq762807
> 
> - Update to version 84.87+git20200110.2d02f07:
>   * Tweaks to make rpm-ndb build
>   * Detect name of coreutils package and don't remove it
> - restore correct version

installed version: 84.88+git20190716.5a0e034-1.1
candidate version: 84.87+git20200110.2d02f07-1.1

I have set "solver.dupAllowDowngrade = true" in zypp.conf.

If I run "zypper dup -D -y --details, I get this:

> ...
> The following package is going to be downgraded:
> post-build-checks
>  84.88+git20190716.5a0e034-1.1 -> 84.87+git20200110.2d02f07-1.1
>  noarch
>  openSUSE-Tumbleweed-Oss
>  openSUSE
> ...
> 544 packages to upgrade, 1 to downgrade, 10 new, 3 to remove, 1 to change arch.
> Overall download size: 0 B. Already cached: 913.1 MiB. After the operation, additional 281.8 MiB will be used.
> 
>    Note: System reboot required.
> Continue? [y/n/v/...? shows all options] (y): y
> 
> Checking for file conflicts: [...........done]

So, zypper handles this situation just fine, and finds no other conflicts.

Arguably the PackageKit zypp plugin should be able to deal with this rather
trivial conflict automatically. But that's not my main point: The real problem
is that the error didn't propagate to GNOME software, which thus offers the
same update again and again, providing no clue to the end user that something
went wrong, let alone what it was.