Okay, so the UUID is now used as intended, and it matches the UUID shown in petitboot itself, but the initramfs cannot find it. There is no other drive attached, which could contain the same UUID, right (well, petitboot does not show any other, so obviously yes)?
I wonder whether we should switch to PARTUUIDs in general, which do not require an initramfs at all. But since initramfs-tools is used, which adds all relevant modules, it shouldn’t matter, and it does work well here on my Odroid N2+, of course.
It shouldn’t matter since it is the initramfs which fails to find the filesystem, not systemd. The only relevant entry is the rootdev kernel command-line entry in /boot/dietpiEnv.txt, which seems to be correct.
Just to rule it out. Could you check the PARTUUID of the partition:
lsblk -no PARTUUID /dev/sda1
and then replace the UUID=... part in /boot/dietpiEnv.txt with PARTUUID=... and the PARTUUID appended accordingly?