Very much what the title says. The new twist is that I am aware of this and of this and of this discussions, and relative replies. But…
me@me:~$ sudo apt-get update
Hit:1 http://fr.archive.ubuntu.com/ubuntu jammy InRelease
Hit:2 http://fr.archive.ubuntu.com/ubuntu jammy-updates InRelease
Hit:3 http://fr.archive.ubuntu.com/ubuntu jammy-backports InRelease
Hit:4 http://security.ubuntu.com/ubuntu jammy-security InRelease
Reading package lists... Done
me@me:~$ sudo apt-get dist-upgrade
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Calculating upgrade... Done
The following packages have been kept back:
tzdata xdg-utils
0 upgraded, 0 newly installed, 0 to remove and 2 not upgraded.
In other words, I do not have conflicting package repos, as suggested in ref. 3, and I have used apt-get dist-upgrade, as suggested in the first two references. Of course
I can update manually,
me@me:~$ sudo apt-get install -y tzdata xdg-utils
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following packages will be upgraded:
tzdata xdg-utils
2 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
Need to get 397 kB of archives.
After this operation, 48,1 kB disk space will be freed.
Get:1 http://fr.archive.ubuntu.com/ubuntu jammy-updates/main amd64 tzdata all 2022c-0ubuntu0.22.04.0 [335 kB]
Get:2 http://fr.archive.ubuntu.com/ubuntu jammy-updates/main amd64 xdg-utils all 1.1.3-4.1ubuntu3~22.04.1 [61,9 kB]
Fetched 397 kB in 0s (1.462 kB/s)
Preconfiguring packages ...
(Reading database ... 346424 files and directories currently installed.
Preparing to unpack .../tzdata_2022c-0ubuntu0.22.04.0_all.deb ...
Unpacking tzdata (2022c-0ubuntu0.22.04.0) over (2022a-0ubuntu1) ..
Preparing to unpack .../xdg-utils_1.1.3-4.1ubuntu3~22.04.1_all.deb ...
Unpacking xdg-utils (1.1.3-4.1ubuntu3~22.04.1) over (1.1.3-4.1ubuntu1.22.04.1) ...
Setting up tzdata (2022c-0ubuntu0.22.04.0) ...
Current default time zone: 'Europe/Paris'
Universal Time is now: Fri Sep 2 07:52:12 UTC 2022.
Run 'dpkg-reconfigure tzdata' if you wish to change it.
Setting up xdg-utils (1.1.3-4.1ubuntu3~22.04.1) ...
Processing triggers for man-db (2.10.2-1) ...
Thus this is a nuisance more than a problem, but: first, it happens rather often, despite my attempts at clearing caches, and so on (apt-get clean && apt-get autoclean), and second I would like to know what causes the problem, because the fact that the update does not take place even after an apt-get dist-upgrade clearly makes me suspect there might be more than meets the eye. Cheers, and thanks.
Best Answer
If these are Phased Updates, then by forcing the installation you became a beta-tester.
Such packages have limited distribution, intended for some users to get first in order to report a broken package. It's possible, but not advised, to force the installation of these packages.
To test if a held back package is phased, run
apt-cache policy <packagename>
and look for the 'phased' percentage, that looks like "(phased xx%)".