Apple has changed the method that allows, in case of problems, to restart or force the restart of the device.As explained in a technical support document, the methods to be followed are as follows:
It must be borne in mind that Apple to force the restart on iPhone 8, refers with standby/reactivation to the side button.
For the traditional shutdown, just hold down the side button for a few seconds until the "Scroll to turn off" option appears.