How to reproduce:
0. On some computers it does not appear. But on those where it does, it can be reproduced consistently.
1. Install and run Process Hacker with the kernel-mode driver. As soon as the driver is unloaded, the bug disappears.
2. Do a lot of things to overload the system. CPU-bound overloading is best.
3. winlogon.exe will enter a peculiar wait state, and won't exit from that state until the kernel mode Process Hacker driver is unloaded.
While in that wait state, winlogon.exe will not respond to shutdown/reboot requests, nor to Ctrl-Alt-Del.
Sorry for not being able to point directly to which component of the driver does this, and I don't think crash dumps are of any use (since they only capture the user-mode side, which has no relevance to the manifestation of this bug)
System: Windows 10 Anniversary Update (on older ones the bug was not present), EFI, Intel Core i3-5005U CPU (on both laptops where I saw it happen). System is up-to-date as of this writing (25.12.2016 15:54 UTC+2)