255 (0xFF) isn't an officially supported key code, and looks like a special value. It suggests that the keyboard doesn't output standard key codes for Power and WakeUp, but relies on a custom driver to handle these keys. Clavier+ cannot do much about this, unfortunately.
The Power and WakeUp buttons of my keyboard are both detected as #255, and when assigned as a shortcut, Supr and they arrow keys also trigger them
Thank you