Power button behavior on Dell XPS

A hold-for-4-seconds shutdown is usually motherboard driven (which is why it works at any time and even if your OS completely locks).

My guess is that the power button was configured to do nothing previously, and by reinstalling Windows it has defaulted to powering down the machines. As this is now a Windows driven action there is no delay.

Have a look in the power profile settings of your machine, you probably need to just change the behaviour of your power button to be able to restore your previous configuration.

To do this, from Control Panel access Power Options.
Look in Properties then Advanced.
Under Power Buttons, select whichever option is most suited to your needs.