unattended-upgrades does not install upgrades on shutdown

[Yvan Masson <yvan@masson-informatique.fr>]

Hi list,

I configured some Buster desktops in a school to upgrade automatically 
on shutdown via unattended-upgrades. I am almost sure it worked at first 
but it does not anymore. I would really appreciate any suggestion as I 
already spent a few hours on this issue without any result.

Symptoms are: shutdown is long (more than 2 minutes), plymouth screen 
shows that unattended-upgrade is running, but no packages are upgraded.

I created /etc/apt/apt.conf.d/51unattended-upgrades-local to store my 
settings (/etc/apt/apt.conf.d/50unattended-upgrades has not been modified):
    Unattended-Upgrade::Origins-Pattern {
      "origin=Debian,codename=buster-updates";
      "origin=Debian,codename=buster,label=Debian";
      "origin=Debian,codename=buster,label=Debian-Security";
    };
    Unattended-Upgrade::MinimalSteps "false";
    Unattended-Upgrade::InstallOnShutdown "true";
    Unattended-Upgrade::Verbose "true";
    Unattended-Upgrade::Debug "true";

I created /etc/apt/apt.conf.d/10periodic:
    APT::Periodic::Unattended-Upgrade "1";
    APT::Periodic::Update-Package-Lists "1";
    APT::Periodic::Download-Upgradeable-Packages "1";
    APT::Periodic::AutocleanInterval "7";

For /var/log/unattended-upgrades/unattended-upgrades-shutdown.log, see
https://paste.debian.net/1106125/
For /var/log/unattended-upgrades/unattended-upgrades.log, see
https://paste.debian.net/1106126/
/var/log/unattended-upgrades/unattended-upgrades-dpkg.log is empty.

As you can see, many upgrades are available. Those packages have already 
been downloaded. Where could be my mistake? Could this be a bug of the 
script when too many packages are upgradable (because yes it has worked 
at the very beginning)?

Best regards,
Yvan