My phone shut off at 43% then said low battery

My second 6s used to do this.

Calibrating the battery didn't resolve the issue for me. I eventually went to Apple for this issue and they replaced the phone. Current 6s hasn't done this at all (although it has a blemish under the screen so I'll have to replace it for a third time at some point...).

 

They replaced mine under warranty, yes.

Granted, mine had done it several times over a few months, so it couldn't be explained as a fluke or because of temperature issues.