I have a Group Policy setting in place that forces all workstations in the same OU checks and installs any available updates from a local WSUS automatically at 3:00 am every single day.
It has worked beautifully until recently I started upgrading them gradually to Windows 10. The ones graduated from Windows 7 to Windows 10 don’t reboot themselves anymore after any security updates.
Turns out, there is another new policy that needs to be enabled to force a reboot immediately after Windows Update installs important updates. It also lets you set up a restart timer, ranging from 15 to 180 minutes, to give users the opportunity to save their work. When the timer runs out, the restart will proceed even if the PC has signed-in users.
The policy is named Always automatically restart at the scheduled time and located at the following Group Policy location:
Computer Configuration > Policies > Administrative Templates > Windows Components > Windows Update