By design, Phorus products will remain on until you tap the power button. Tapping again when powered off, will power the unit back on. You do not need to press and hold, and simple tap will do the trick, but you can also disconnect the units from power by unplugging them, and they will safely shutdown upon detecting power interupt.
The Phorus PS5 and Phorus PR5 products have an extra function that allows them to automatically power off a little while after inactivity. This mode is indicated by an Orange power light, whereas always on mode is indicated by a White light.
To switch between these modes, press and hold the power button for 5 seconds, until you see its light change from one color to the other.