I'll answer my own question:

The only necessary fix was to append log volume mount point to RequiresMountsFor in systemd-journal-flush service's unit file:

RequiresMountsFor=/var/log/journal /log

In yocto, that required patching systemd recipe to make a change in units/systemd-journal-flush.service.

After this change, new boot is correctly recognized every time, and no more unmount failures during poweroff.