The mantic pi images (both server and desktop) have pre-generated machine-id values in /etc/machine-id and /var/lib/dbus/machine-id. In livecd-rootfs this is taken care of with a live-build hook [1][2] but I don't believe there's an equivalent in the current ubuntu-image implementation (or none that I could find).
More seriously, the mantic pi server images also have pre-generated SSH host keys. Thankfully, the default cloud-init configuration regenerates these on first boot before SSH comes up, but again these should not be shipped in the images, and again this was taken care of previously with a live-build hook [3][4].
[1]: https://git.launchpad.net/ubuntu/+source/livecd-rootfs/tree/live-build/auto/config#n1238
[2]: https://git.launchpad.net/ubuntu/+source/live-build/tree/share/hooks/004-remove-dbus-machine-id.chroot
[3]: https://git.launchpad.net/ubuntu/+source/livecd-rootfs/tree/live-build/auto/config#n1239
[4]: https://git.launchpad.net/ubuntu/+source/live-build/tree/share/hooks/006-remove-openssh-server-host-keys.chroot
I should add a note of thanks to ^AT^ on the #ubuntu-server channel who first pointed out the issue with the pre-generated machine-id, which then led me to investigate what else was not being reset by the new image build process.