In summary, with Fast Startup the current user is logged off but then instead of shutting down the operating system as expected the current state of Windows is written to the hibernation file. Then when the system is restarted the state is resumed and the user logged back in.
The result is that Windows doesn't really get restarted, just the user is logged off and back on.
To force a complete Windows shutdown and restart, hold SHIFT while you select Shutdown or Restart.
I encountered this recently when I encountered the DHCP problems Windows 10 introduced. A Windows restart using Fast Startup wouldn't clear this.