My phone does this as well.
Reason?
Because the Galaxy S6 is out. Ever notice the photo quality starts to downgrade over time? Regardless of how well you take care of your phone? Every update done to these phones, the older they get (and by older, I mean anything older than the newest version), they get a little worse, until you're forced to upgrade to the newest.
Irritated at this point. Just because newer technology happens, doesn't mean older technology should get worse... it just shouldn't be as good as the newest... right?
I turn my phone off, turn it back on, reset it, etc... goes away for a few days, then comes right back. Shameful Samsung, just shameful.
I imagine it will continue to do this until I upgrade to the newest Galaxy available... which won't have it's own little quirk, until a newer Galaxy comes out.