This is touch ic disease, 100%. I wrote a blog post about this recently that explains the underlying design flaw in the iPhone 6+ that leads to this issue. The only solution is to have the touch ics replaced on the board by a microsolderer.
http://www.mendonipadrehab.com/entries/general/the-epidemic-of-iphone-6-6-touch-ic-failure-explained-apples-continues-to-pull-the-wool-over-the-consumer-s-eyes