iOS 11.4 and 11.4.1 have battery related bugs. My dad's iPhone SE (just a few months old with a 100% fresh battery) goes from 100% to 50% in 1 hr regular usage with 10 hours standby. My mom's iPhone 7 is just a bit better.
My iPhone X suffered on 11.4 too, but not as severely. iOS 12 Beta 3 is much better but not perfect.