Same thing here but I noticed, at least with me, it's been when I hit the camera app which is above my home button.
I was going to take it back but then, at night when it was a bit darker, the light was off and after a few secs came back on.
I'm thinking its some kind of fail safe and not a defect now. Maybe I'm wrong but it's so consistent that I can control it and know what to do when it happens and I'm ok with that. At least for now.