Comment by da_chicken

It's usually referenced as "Hybrid Shutdown" in Microsoft documentation.

https://learn.microsoft.com/en-us/windows/win32/power/system...

> "Note: In Windows, fast startup is the default transition when a system shutdown is requested. A full shutdown (S5) occurs when a system restart is requested or when an application calls a shutdown API."

Technically it's entering a "hybrid" S4 Hibernation with S0 Standby after all users have been logged out. To bypass it you need to press Shift while clicking Shutdown, running the `shutdown /s /t 0` command, or else disabling Fast Startup. You can tell that you didn't do a true S5 shutdown because the system's uptime will not reset.

But disabling Modern Standby in your BIOS will also disable it because Window's power management logic is set during installation. With modern standby enabled, Windows tries to be always on and always connected. When you disable modern standby, Windows doesn't entirely change it's logic so much as it notices it can't send the same power state commands, so it reverts to S5 Shutdown.

I chose to disable it in BIOS because Microsoft can't really turn it back on when I do it that way. Because the thing is... I disabled Fast Startup after the second time it happened. But some Microsoft updates re-enable Fast Startup, and it's not hard to find forum posts complaining about that.