I bought a brand-new Dell Alienware laptop with Windows 11 Pro 21H2 (OS Build 22000.1219), and Screensaver, Sleep, and Hibernate have never worked, out of the box. I have tried every suggestion in this 6-page thread, except "reinstall Windows" and nothing has ever worked, not even once. Windows 11 is definitely still not a business class operating system.
Update 1:
I ran the upgrade to Version 22H2 (OS Build 22621.819) which is the equivalent of "reinstall Windows" and it had no effect. Screensaver, Sleep, and Hibernate still do not work. 12 months now.
Update 2:
I set my screensaver to 1 minute, and put the device to sleep after 2 minutes, then tried unplugging everything 1 at a time waiting 5 minutes until there was nothing left plugged in, not even the ethernet cord, external speakers, power adapter, external monitor, nothing, just the bare laptop sitting by itself, and it will just sit there displaying the desktop until the battery is fully drained.