Was this using the information in the Battery section or in the Screen Time section?I hope it's inaccurate, because if it isn't, then iOS 11 and 12 absolutely destroyed the battery life of a friend's iPhone 7. I knew battery life had decreased with iOS 11 (he was getting 5-6 hours on iOS 11... I'm getting 8-9 with my 6s on iOS 9).
I checked my friend's battery life on iOS 12, and it was - according to a manual add-up of Screen Time - 2.5 hours of use (both on and off screen, equal to the pre-iOS 12 stat), with 25% battery life left. I hope that isn't correct because it would mean that iOS 12 is far worse than iOS 11 in the battery life department, and iOS 11 was bad enough already.