It sounds like MAAS is installing Ubuntu on the machine fine, but the resulting OS cannot boot, which is typically a sign that the wrong boot material/location was used.
In my experience, if the machine was originally commissioned under legacy BIOS settings, it will need to be recommissioned with the UEFI BIOS settings enabled in order for MAAS to recognize the change. You should be able to retain storage and network configurations if desired, though the subsequent deployment may need specific partitioning for UEFI (such as the small fat32 partition for /boot/efi).