How long ago did you have a new 8+?
When it came out, maybe I should of said that, but what I meant to say, is that from my understanding, it's a defect, and doesn't have to do with the case or how you hold it.
Apple wasn't able to figure out what the issue was, so went to the next room, and called me, when they weren't able to hear me, they swapped it out on the spot.