Third N910T replacement; something stealing app focus

I'm about to receive my third warranty replacement for the numerous problems I had with my Note4. Most of the problems are fixable (camera, GPS, etc), but one problem is bad and I don't see a simple solution.

Something seems to be intermittently stealing the focus from my app (I believe). So, if I'm watching YouTube, it randomly pauses because it thinks I've switched apps. If I'm recording video, it randomly stops, because it thinks I've switched apps. If I'm playing a video game, it pauses while I'm playing until 'I return'. If I go to the app-switcher, sometimes it closes randomly before I select which running app to switch to.

I actually bought a new SD card (64GB wasn't really doing it for me, anymore, anyways) but the problem persists. The phone does seem better behaved before all my apps repopulate (although there's not much to do on the phone before the apps are back), so I suspect it's an app, but I haven't heard anything about this sort of problem on any website or forum.

Lastly, the problem may have started around the time of the StageFright patch in August, but a lot of apps also updated around that time.

I'm stock without root (maybe in the future), although I always disable T-Mobile bloatware like DT_Ignite. Any help or suggestions are appreciated. Thanks in advance.

Dual camera won't focus, makes crackling sound (water damage?)

Last Tuesday my phone got wet while on a bicycle ride where it started to rain. The phone got wet enough that it started to randomly jump to different screens. I turned it off and put it in my pocket. Once I got home and dried the phone it seemed fine, and in fact the next day used the dual cameras to take a pic with no problem. There seems to be no water damge indicator in the dual flash.

Today tried to take a picture using the dual cameras and immediately got this crackling sound and the cameras wouldn't focus. I've done a search and couldn't find this exact problem elsewhere. The best way to understand what the cameras are doing is to show a video taken with them:

https://youtu.be/rms54f8O4vA

You can both see the inability of the cameras to focus and hear the crackling sound they make while trying to focus.

If I shift to the front facing camera it works just fine.

What to do? are the dual cameras kaput? :(