I bought a new iphone 11 sealed on July 16 and until Monday evening my battery performance was naturally 100%, this Wednesday morning by chance, I check and the status has changed to 86%. It's just inexplicable, some forums say it's due to upgrading to ios 14.5 but I installed this update as soon as it was released and had no bugs.
Make sure you update to 14.7.1. Then go to Settings/Battery and you should see a notice that it needs to recalibrate the battery capacity measurement. See→About recalibration of battery health reporting on iPhone 11 series with iOS 14.5 and later - Apple Support
Did you buy the phone from Apple or an authorized reseller? If not you can check that it really is new here→Apple - Support - Check Your Service and Support Coverage
Bug with battery status
