Check wake-up settings in BIOS. Disable every wakeup trigger.when I turn off the PC it automatically turns on.
It was already turned off.Is this windows 10?
if so, try turning fast startup off. It might help as I don't think win 10 has a way to turn itself on again after its shutdown with this turned off.
Right click start
choose Power Options
Under related settings, choose Additional power settings
in control panel, click "choose what the power button does"
Next to Blue/yellow shield, click "Change settings that are currently unavailable"
Under shutdown settings, untick Turn on fast startup and then click Save changes.
Glitch is after info in this screen, maybe what it shows in the drop down next to When I press the power button - default is shutdown
If it still starts with Fast startup turned off, it has to be hardware as Windows is actually off with fast startup turned off, and you turn it off.
A Win 10 pc with fast startup turned on isn't actually off when you shut it down. it goes into a hibernate state and saves info about what was running into hiberfil.sys which is 1st thing loaded when it starts up again.
It happens too when I use the windows shutdown.So ok to be clear, are you just hitting the power button once and not holding it down when you go to turn it off. Because one tap may just be putting it into hibernation, which would allow it to wake up by an update etc. Holding the power button down for a few seconds turns it totally off.