After discovering other issues that made the live installer not functional (See https://bugs.launchpad.net/ubuntu/+source/livecd-rootfs/+bug/1847835) I gave up and went back to the NON Live ISO, which let me create a mirrored root as expected.
After discovering other issues that made the live installer not functional (See https:/ /bugs.launchpad .net/ubuntu/ +source/ livecd- rootfs/ +bug/1847835) I gave up and went back to the NON Live ISO, which let me create a mirrored root as expected.