However, what's different is that the customer has enabled UCA[1] on Focal(sudo add-apt-repository cloud-archive:yoga), so the current installed version of libvirt-daemon-system is 8.0.0-1ubuntu7.6~cloud0, instead of the original 6.0.0-0ubuntu8.16 in Focal. 

The customer confirmed that this issue cannot be reproduced on Focal machine without enabling UCA. I believe the fix starting from "6.0.0-0ubuntu8.10" shoud fix the issue.

I'm unclear whether this issue would also occur with the Jammy version, as the current version of libvirt-daemon-system on Jammy is 8.0.0-1ubuntu7.6. 

$ rmadison libvirt-daemon-system -a amd64|egrep  'focal|jammy'
 libvirt-daemon-system | 6.0.0-0ubuntu8    | focal           | amd64
 libvirt-daemon-system | 6.0.0-0ubuntu8.16 | focal-security  | amd64
 libvirt-daemon-system | 6.0.0-0ubuntu8.16 | focal-updates   | amd64
 libvirt-daemon-system | 6.0.0-0ubuntu8.17 | focal-proposed  | amd64
 libvirt-daemon-system | 8.0.0-1ubuntu7    | jammy           | amd64
 libvirt-daemon-system | 8.0.0-1ubuntu7.5  | jammy-security  | amd64
 libvirt-daemon-system | 8.0.0-1ubuntu7.6  | jammy-updates   | amd64
 libvirt-daemon-system | 8.0.0-1ubuntu7.7  | jammy-proposed  | amd64


Or should it be said that this problem is not just an issue with libvirt-daemon-system itself, but also involves a compatibility issue with AppArmor?

Again, the following pkg versions combination on Focal 20.04.6(enabled UCA) can reproduce the issue.

ii  apparmor                              2.13.3-7ubuntu5.2                     amd64        user-space parser utility for AppArmor
ii  gir1.2-libvirt-glib-1.0:amd64         3.0.0-1                               amd64        GObject introspection files for the libvirt-glib library
ii  libapparmor1:amd64                    2.13.3-7ubuntu5.2                     amd64        changehat AppArmor library
ii  libvirt-clients                       8.0.0-1ubuntu7.6~cloud0               amd64        Programs for the libvirt library
ii  libvirt-daemon                        8.0.0-1ubuntu7.6~cloud0               amd64        Virtualization daemon
ii  libvirt-daemon-config-network         8.0.0-1ubuntu7.6~cloud0               all          Libvirt daemon configuration files (default network)
ii  libvirt-daemon-config-nwfilter        8.0.0-1ubuntu7.6~cloud0               all          Libvirt daemon configuration files (default network filters)
ii  libvirt-daemon-driver-qemu            8.0.0-1ubuntu7.6~cloud0               amd64        Virtualization daemon QEMU connection driver
ii  libvirt-daemon-driver-storage-rbd     8.0.0-1ubuntu7.6~cloud0               amd64        Virtualization daemon RBD storage driver
ii  libvirt-daemon-system                 8.0.0-1ubuntu7.6~cloud0               amd64        Libvirt daemon configuration files
ii  libvirt-daemon-system-systemd         8.0.0-1ubuntu7.6~cloud0               all          Libvirt daemon configuration files (systemd)
ii  libvirt-glib-1.0-0:amd64              3.0.0-1                               amd64        libvirt GLib and GObject mapping library
ii  libvirt0:amd64                        8.0.0-1ubuntu7.6~cloud0               amd64        library for interfacing with different virtualization systems
ii  python3-libvirt                       6.1.0-1                               amd64        libvirt Python 3 bindings



[1] https://wiki.ubuntu.com/OpenStack/CloudArchive