Normally, when I press the power button on my phone, the screen turns off and when I press it again, I'm presented with the lock screen. (I don't have a password or pattern, so it's just the "Swipe up to unlock" screen.)
In the last month or so, my phone has started exhibiting the symptom that I press the power button and the screen turns off, but when I press the power button again, it's not at the lock screen, but whatever the front application happens to be. Sometimes I'll note that about 20 seconds after I press the power button to lock the screen, the phone will emit another sound from the speakers that sounds a lot like the "I've just locked" click.
This doesn't happen all of the time. At a guess, I'd say that about a third of the times that I wake from a black screen, I get no lock screen. Also, I'm not sure if pressing the power button to lock vs. a timeout makes any difference.
It seems to me that there must be some application that's keeping the phone from locking, but I think I've uninstalled all the applications that I installed shortly before this started happening. I'm guessing that it's an application that had been recently updated, but I don't know how to track that down.
I don't really want to uninstall apps one-by-one, as I can't reproduce the problem. It never happens when I want it to.
Does anyone have any ideas how to track down this problem?
This is on a Nexus 5 running 6.0.1, rooted. (I never do anything with root, though, and it's preventing me from upgrading, so I should really undo it.)