I had one of these phones too - it was a bad batch of serial numbers according to Apple. If this happens to you: https://www.apple.com/support/iphone6s-unexpectedshutdown/
Apparently all phones suffer from that problem but the batch we referred to seem to be especially bad. The software update just seems to have mitigated the effects of iPhones which were mostly ok but shut down occasionally (and the effected ones which shut down almost every week when it was at 70% and about once or twice a day at 20-30%).