iPhone Shutting Down Daily

I have an Iphone 6s with iOS 14.6. I keep my phone plugged in and charge at night. Within the last week, every morning when I have taken it off the charger despite the battery saying it is at 100% the phone automatically shuts off and then restarts with only 10% battery remaining. I plug the phone back in and it recharges to 100% within 30 minutes and is fine the rest of the day (so far). I even tried turning the phone off overnight (while plugged in), but this still occurred when I turned the phone back on in the morning.

Is this a bug in the latest iOS update?