iOS Update May Affect iPhone Battery Health, But Don't Panic! 

According to recent reports, the iOS 18.1 update may cause a drop in iPhone battery health, but there's more to it than meets the eye. The Maximum Capacity number displayed on your device is not a measure of overall wear and tear, but rather how much power the battery can hold relative to its new state.

Source: https://www.zdnet.com/article/ios-18-1-update-worsen-your-iphone-battery-health-dont-let-this-spec-fool-you/