Comment 5 for bug 1813394

Digging even further back, it turns out that IFDOWN is in itself a workaround for an equal issue dating back to ~2012. As such, the core of the issue lies with how initramfs network setup overrides netplan. Adding that package.

Given the very limited setup /etc/initramfs-tools/initramfs.conf allows (including the DHCP default mentioned in the description), there is a case to be made for allowing admins to rely on the full config rather than initramfs for their network setup.