My motion sensor is working fine in the sense that when I turn "off" the tv it goes to art mode and shuts down after 15 min of no motion (I have set it to 15 min in settings) but when someone goes into the room (say late at night) the TV restarts on my cable box (HDMI) not art mode. I have tried with "Auto run last app" toggled on or off and either way it turns on with the cable box (tv show and sound) vs art mode.
Strange. On my Frame, it starts back on art mode when it wakes up either from detecting motion or exiting out of night mode.
Perhaps it could be the CEC command from your cable box kicking your TV out of art mode?