What archlinux says about systemd + journal (here: https://wiki.archlinux.org/index.php/systemd#Journal)
"If the journal is persistent (non-volatile), its size limit is set to a default value of 10% of the size of the respective file system."
Which is perhaps an unfortunate way to try to avoid filling the disk, but it is what they went with.
So despite any corruption or renaming issues, this, at least from the FW person doesn't seem to be true:
Journal logging and rotation are working well, and journald always keep 10% free on root partition (default value).
The below worked for me -- note the addition of the sync step before reboot. Without that the /etc/machine-id file was not flushed to the flash for me.
# umount /etc/machine-id
# systemd-machine-id-setup
# rm -rf /var/log/journal/*
# sync
# reboot