I have an iPhone SE running iOS 9.3.3. Once in a while, it seems like the screen doesn't automatically turn off when it should. It should auto-lock (screen turns off) after 3 seconds on a lock screen, 15 seconds if media is playing on the lock screen, or one minute (based on the settings) when the phone is in normal use, with the exception of playing media or something that would prevent the screen from shutting. It normally does this just fine, but not always.
So for example, this morning I noticed my phone screen was still on, on the lock screen, after a notification on the screen was already 2 minutes old. I tried doing all kinds of things to see if the screen would start auto-locking properly, I put it in airplane mode, closed all background apps, tweaked auto-lock settings, closed all Notification Center widgets, removed all Notification Center notifications, tried playing some music, tried using the camera, tried covering the light sensor on the phone, tried cleaning the screen. Nothing helped. Then I rebooted the phone and it is working properly again. (If I recall, the problem doesn't persist if I just manually lock the screen and ignore it, it gets back to working normally later on, but rebooting fixed it right away.)
But this shouldn't happen in the first place. It is especially bad like if I have my phone in my pocket and I get a notification and the screen has just been running for a half hour before I check the phone. And this could happen. One time I looked at my phone and it was just on, sitting on the bed, with a 30 minute old notification on the screen. Not good for battery usage among other things.
There's nothing unusual about what I do with the phone, I have it on rotation lock, I have a phone case, I from time to time use it on VoiceOver (but VoiceOver was not on when I had this problem), screen is usually near the dimmer side. I don't know what else might be useful in diagnosing this. So what could possibly be causing this?
Edit: In the few months since upgrading from iOS 9.3 to 10.3 I stopped noticing this.