I didn't know this was a widespread problem. I had the problem start about a month ago on a iPhone 6S Plus. The "genius" ran their battery diagnostic and said it was within spec. Their suggestion was to make an appointment if it happens again. I told them it had been happening every day for a week, and asked what would be the difference in my next appointment? They don't have an answer for these type of questions since they are just trying to get you to leave. I also questioned why the battery diagnostic was all they were going to look at. I mentioned it could be numerous things - the battery, the battery cable, the connection on the board for the battery, etc. and that running their diagnostic doesn't test for all the hardware involved. I left frustrated.
That night when my phone cut off, I was able to grab another phone and take a video. The device powered back up with 40% battery and then shutdown again a few times. When I showed the video at the next appointment, they replaced the device.