I purchased a Frame recently as well and experienced something very similar. I was able to get the motion detector to work by shutting off the light sensitivity (night mode) in the very next setting in art settings. It seems that the lighting in my room was low enough to override the motion detector by putting the tv into sleep mode. It was evening time and somewhat low ambient lighting in the room while troubleshooting. I was surprised this worked as it wasn’t very dark in my room, Samsung should adjust the threshold of what constitutes night mode.
Let us know if shutting off night mode worked for you.