libvirt users - Mar 2018 - Can't shutdown/reboot Windows 10 guest if on lock screen

I am running Windows 10 vms on a Linux host with libvirt 4.0.0 and qemu 
2.10.2

I can shutdown and reboot the Windows 10 guests if the no user is logged 
in the vm, or if the user is logged in and active/seeing the desktop. 
But after a while, when the lock screen kicks in (but the user is still 
logged in technically), the shutdown and reboot commands don't have any 
effect any more. Is there any way around this (except for disabling 
automatic screen locking on Windows)?

Thank you for any hints.