Comment # 4 on bug 1055641 from Franck Bui

I'm wondering how you can deduce from the screenshot that this is happening
after all file systems have been unmounted.

Olli, can you try to enable the debug log right before shutting down your
machine ?

You can do that by issuing "systemd-analyze set-log-level debug".

Once done, restart the machine and attach the outpout of "journalctl -b -1"
which should show the logs from the previous boot.

You can also find some additional info on how to debug the shutdown process
here: https://freedesktop.org/wiki/Software/systemd/Debugging/#index2h1

Thanks