As Fox_exe points out, the device cannot shut itself “off.”
Instead, the shutdown command tells the system to halt everything, write unwritten data to the disk, unmount, and then just sit there until you interrupt the power.
If you want to reboot the unit, tell it to reboot instead of shutdown.
It’s a shame this system does not have a free gpio, or a clean way to attach to one if it did. Using a simple flipflop circuit that allows power through as long as a certain GPIO remains low, (and stays powered even when the flipflop disables power, until a manual high state is issued with a button) would implement a makeshift power button, and a means to allow the unit to actually shut itself off on shutdown.
As is though, you can tell the unit to shutdown using ssh before yanking the power, if you need to move the unit.