Q: iPhone 6 shuts down at auto-lock

My husband and I each upgraded to the iPhone 6 in November. We have had zero problems until yesterday.

His phone shuts down automatically when the 5 minute auto-lock activates. I reset the phone "as new" but this has not solved the problem.

Prior to performing the reset, I turned on the phone and charged the battery as it had dropped to around 9% from 78% even though the phone was off for approximately 9-10 hours. I noticed that when the auto-lock feature was set, the phone would power off at the 5 minute mark, despite being charged. Once I turned off the auto-lock feature, the phone was able to charge to 100% and did not shut down on its own. We removed the phone from the charger and used it without incident. When the phone was not in use again, it shut down again.

Any recommendations are welcome!