I also am having that problem my iPhone 6s plus will shut down almost as soon as I set it down every time. Except when it is plugged in it will stay on all night /day. This started after screen replacement. And I took it into have it fixed yesterday it's not battery they say. Yet they say they can't figure it out either. So basically told me good luck. So I am going to remove screen again and see if there is a loose connection somewhere. But just letting you know it is happening to these phones apparently.