Roku devices don't turn off. The power button on the remote is strictly for the TV. The light goes off when the screensaver activates or the Roku device goes into "power saving" mode (if the device supports it). You can pretty much ignore the light since it doesn't mean much and seems to be an unreliable indicator.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee.