Yup! well seems only when I'm playing, not in the home environment. I've think just found the setting that's causing the problem. If I go to the Library then select Oculus TV there's an Unknown Sources>Settings option in there if I select Display and there's a sleep option there which is set to 30mins if I set to 15secs then I get sleep mode in 15 seconds even while playing, hitting the power button to wake, I can't find anyway to disable this.
It seems they are overwriting the default setting, maybe you have installed something or opted as developer or have installed sidequest, i not have such that is why i not see it on mine.