its a bug in android do this go to settings/display choose screen timeout pick anyone, reboot your phone, pick the one u want reboot you phone should be fixed.
Actually, I changed the screen timeout about 2 days ago and it did nothing. But I just rebooted for the hell of it and it was fixed. So I did the exact 2 steps you gave, just over 2 days. lol.
Kind of a noob still about some aspects. My screen has just stopped timing out. It will stay lit up until I turn it off manually with the button. The timeout option is set for 30 seconds in settings. I dunno. Any ideas?
Yoy must have installed an app that has a setting 'prevent phone from sleeping'. Check each app and its settings one by one and you'll probably find the one causing your problem.