Even after the update to iOS 15.4.1, some iPhone owners have unfortunately suffered from a few annoying problems with the synchronization of iMessages as well as the location services in some of Apple's stock apps, which apparently do not work as intended for users in at least one region.
https://www.notebookcheck.net/iPhone-users-complain-about-several-issues-after-iOS-15-4-1-update.612646.0.html
The claim that iOS 15.4.1 corrected the battery drain problem is false. Problem continues under 15.4.1 on 2 iPhone 11s and 4 iPads.
Iphone battery drains in 5-6 hrs in any mode after installing ios 15.4 or 15.4.1 or 15.5, a new battery does not help, factory reset does not help either. Switching the battery to a unit with ios 14 the battery works perfect. Apple must have deliberately plantet a bug in ios 15.4 and above. This indicates to stay away from Apple products in the future, zero respect for customers.
It was similar for my iPad Mini 4 of 2015 with an update in early 2016 decreasing battery life by 15.5% and later never retracting this.