Note that I'm explicitly asking for the devel release (-d).
If I do not install lubuntu-desktop (step 3) then the upgrade works, so this looks like somehow specific to lubuntu. This is confirmed by the fact that in automation upgrades done by auto-upgrade-testing work fine.
I can reproduce this by:
1. Starting a Kinetic LXD container
2. apt update && apt full-upgrade
3. apt install lubuntu-desktop
4. do-release-upgrade -d
Note that I'm explicitly asking for the devel release (-d).
If I do not install lubuntu-desktop (step 3) then the upgrade works, so this looks like somehow specific to lubuntu. This is confirmed by the fact that in automation upgrades done by auto-upgrade- testing work fine.