YouTuber iAppleBytes set about getting to the bottom of this by installing the iOS 18.6 final update on a raft of devices to ...

iPhone 15: Underperformed, likely due to iOS 18’s impact on battery efficiency. iPhone 13, 12, and 11: Predictable decline, with the iPhone 11 showing the shortest battery life.

However, the battery drain wasn't fixed, 4hours and 20minutes it drained 11%, from 70% to 59%. From kirbysmartdawg: Clearly, something isn't right on Apple's end. The idle battery drain is excessive.

Ever since upgrading to iOS 18, my iPhone's battery life has taken a serious hit. Things only got worse with subsequent releases, and nothing has been improved by the latest 18.3.1 security patch ...