I have this problem as well. The joke is, my iPhone 6 was an original run purchased that September. The crescent shift did not materialize, what happened instead was tiny specks of dust appeared between the camera lens and the glass. I ignored it for a while and then happened to ask about it while i was in the Apple store. The concierge told me that it should be fixed and she made a genius appointment for me on the spot. The tech checked it out and said they would replace the display. When we told me how long it would take, i balked a bit since i was paying for parking by the hour. When i i hesitated, he said, "you know what, i know you're a good customer, so i'll just replace that for you right now."
So that was very nice of him. And then my replacement phone almost immediately 'crescented'

It's getting worse, too. It is almost 1/3 of the way over now.