I’ve tested this again with the new desktop ISO and although the installer seems to complete successfully, after a reboot the device always asks for the recovery-key. Which I obviously cannot retrieve, since you can only get it from the booted system.
Manual installation works fine, but I have tested autoinstall on a VM and physical laptop and both fail as above.
For anyone else having the same issue, I’ve opened a bug report here: https://bugs.launchpad.net/ubuntu-desktop-installer/+bug/2039557