Hi @ludovic,
The teefix described on github worked for me! Good find, kudos!
So I took a random debian-12-minimal-based template and removed the " | tee…" in /usr/lib/qubes/upgrades-installed-check as described in the git issue. Powered off the template, started a qube based on that template, and 5 minutes later… boom, “Updates Available”.
Thanks for looking into the issue!