Comment 2 for bug 1971165

I've now managed to recreate this in a checkpointed virtual machine.

The last few lines from do-release-upgrade are:

Updating repository information
Hit http://gb.archive.ubuntu.com/ubuntu jammy InRelease
Hit http://gb.archive.ubuntu.com/ubuntu jammy-updates InRelease
Hit http://gb.archive.ubuntu.com/ubuntu jammy-backports InRelease
Hit http://security.ubuntu.com/ubuntu jammy-security InRelease
Fetched 0 B in 0s (0 B/s)

Checking package manager
Reading package lists... Done
Building dependency tree
Reading state information... Done

Calculating the changes

Calculating the changes

EFI System Partition (ESP) not usable

Your EFI System Partition (ESP) is not mounted at /boot/efi. Please
ensure that it is properly configured and try again.

There is code in /usr/lib/python3/dist-packages/DistUpgrade/DistUpgradeController.py that matches this message but I put some diagnostic messages in this file and they weren't displayed which confuses me a bit. However that might just be my faulty understanding of how python works.