The rumored Action button may have prompted Apple to create new button press combinations to force restart or reboot your iPhone 15 Pro or iPhone 15 Pro Max.
iPhone 15 Pro may bring new button combinations to power off and force restart
Action Button
How the iPhone 15 Pro’s Action button might work in the Camera app
Depending on the amount of pressure applied to the force-sensitive Action button, you'll invoke different features of the stock Camera app on the iPhone 15 Pros.