Aha! Thanks to all of you for your input. Indeed the etc/zipl.conf seems to be off and apparently Michael already pointed out the relevant code where this can be fixed:
root@ubuntu-server:/# cat target/etc/zipl.conf
# This has been modified by the MAAS curtin installer
[defaultboot]
default=ubuntu
Aha! Thanks to all of you for your input. Indeed the etc/zipl.conf seems to be off and apparently Michael already pointed out the relevant code where this can be fixed:
root@ubuntu- server: /# cat target/ etc/zipl. conf
# This has been modified by the MAAS curtin installer
[defaultboot]
default=ubuntu
[ubuntu] 288689a9- 116a-4867- 93a2-75a963062c 61
target = /boot
image = /boot/vmlinuz
ramdisk = /boot/initrd.img
parameters = root=UUID=