That sounds like the new "feature" that Roku implemented recently to prevent the device from doing what you described -- running for more than a few hours at a time before returning to the Home Screen:
https://forums.roku.com/viewtopic.php?f=28&t=94604It shouldn't happen if you're playing a single video continuously, but I imagine it might if you're looping the same video repeatedly. A possible workaround is for your channel to send ECP key press commands to itself at regular intervals to prevent screensaver activation/returning to the Home screen.
