Shutdown /s works but Windows Shut Down just takes the computer to the Welcome Screen.

So, I've had this problem for about a year, since installing a new Power Supply on my desktop:

When I select Start Menu > Shut Down , it no longer shuts down the computer. Instead, the screen goes black, monitors go off, and seconds later the monitors turn back on and the Windows Welcome Screen appears. This sequence is accompanied by audible revving of fans after the screens go black. There is no display of UEFI splash screens at any point.

If I select Welcome Screen > Shut Down, the same behaviour results.

When this happens, the only apparent option to shut the computer down is to Hard Power Off by holding the Power button down for 5 seconds, which is annoying.

I initially thought it was a problem with the new Power Supply that I had installed. However, Restarting the Computer and Shutting Down after Updating Windows both seem to work as normal.

Also, occasionally, the Shut Down behaviour reverts to normal expected behaviour. This happens very occasionally around the time of Patch Tuesday. But it doesn't happen with every Update. And the correct behaviour doesn't happen for very long before the problem reappears a few days or a couple of weeks later.

Anyway, that proves that Power Down does work in principle with the new PSU. Suggesting that the problem might be somewhere else - but where?

The Windows Power Plan is "Balanced" with nothing changed.

Then I hit on the idea of trying to Power Down via cmd shutdown /s and that works perfectly as a workaround.

Question remains: why does shutdown /s work as normal when Windows Shut Down merely restarts the computer? And, how to fix Windows normal Shut Down?

Suggestions welcome.