subiquity asks to "update" to version older the the one actually installed

Yeah, snapd doesn't have a concept of ordering of versions, it just offers you the version in the channel you are tracking if it is different to the one you are running. How did you end up in this situation? Using livefs-editor to inject an updated snap to the 20.04.2 media?

We are doing snap_preseed as a livecd hook:

snap_preseed $INSTALLER_ROOT subiquity/classic=latest/stable

I have seen that actually something triggers a channel switch by looking at snap changes:

# snap changes
ID Status Spawn Ready Summary
2 Doing today at 15:16 UTC - Initialize device
3 Done today at 15:16 UTC today at 15:16 UTC Change configuration of "core" snap
4 Done today at 15:17 UTC today at 15:17 UTC Switch "subiquity" snap to channel "stable/ubuntu-20.04.2"
1 Done in 48 days, at 08:29 UTC today at 15:16 UTC Initialize system state

And I found in the subiquity log (see attached the full log):

2021-05-27 15:17:38,610 DEBUG root:39 finish: subiquity/Refresh/configure_snapd/get_details: SUCCESS: current version of snap is: '21.06.01'
2021-05-27 15:17:38,611 DEBUG root:39 start: subiquity/Refresh/configure_snapd/switching: switching subiquity to stable/ubuntu-20.04.2

It looks like subiquity is taking the channel from /cdrom/.disk/info, that is

Ubuntu-Server 20.04.2 LTS "Focal Fossa" - Release arm64 (20210714)

in this case. But we don't want that in this case. I guess that the workaround would be to set the kernel command line to "subiquity-channel=latest/stable"?

Yes that will work as a workaround. How are you making your images though? An ISO made on 20210714 shouldn't be labelled 20.04.2 really -- the ones we make at http://cdimage.ubuntu.com/ubuntu-server/focal/daily-live/ aren't (unless I completely forget how this works).

The information in /cdrom/.disk/info comes from debian-cd:

https://bazaar.launchpad.net/~ubuntu-cdimage/debian-cd/ubuntu/view/head:/CONF.sh#L302

We can replace that in the build script, but not sure what should be put there tbh.

We did actually go and implement this, but this sort of "downgrade" does show up in some legitimate cases as well. So yesterday we ended up reverting to the previous logic, which is that whatever is on the tracked channel is considered the version that we should offer.

Marking this Won't Fix, sorry.