I had been affected by the same problem, but the bug doesn't lies in UU.
What is the output of the "users" and "loginctl" commands in a terminal ?
What is your display manager ?
Maybe you are affected by this SDDM bug:
2018-03-06 10:33 UTC+01:00, Colin 't Hart <colinth...@gmail.com>:
> Package: unattended-upgrades
> Version: 0.93.1+nmu1
> Severity: normal
> I have
> Unattended-Upgrade::Automatic-Reboot-WithUsers "false"
> in my /etc/apt/apt.conf.d/50unattended-upgrades
> I have noticed several times now when I arrive at my office in the morning
> that my machine has rebooted even though I was logged in when I left the
> evening before. I use a standard install of XFCE with its screen locker and
> my screen was locked when I left.