Comment 6 for bug 1778219

This is not a bug in unattended-upgrades, but u-u triggers the problem in many packages which are not able to install/upgrade during a system shutdown.

Helper scripts in init-system-helpers must not start services when maintainer scripts are executed during shutdown and maintainer scripts must use helper scripts and not call systemct and friends directly to start services.

Snapd for example calls systemctl start "$unit" in postinst.

Thank you for the bug report. I marked it "Confirmed for u-u" to not let it disappear, but u-u is just triggering the issue and it can't be fixed in u-u.