In brief: iPhone users eagerly anticipated Apple's release of iOS 15.4 for the added ability to use Face ID while wearing a mask. In the days after installing it, however a small number of people reported reductions in battery life. Apple has now issued a fix.
With the release of iOS and iPadOS 15.4.1, Apple says it has addressed an issue that reduced the battery life on some devices after installing version 15.4. The latest update also includes fixes for braille and hearing devices. Even users who didn't have problems with battery life should still install it for the security updates.
Some users reported battery life on their iPhones fell by as much as 50 percent upon installing iOS 15.4. Initially, Apple support tweeted that some problems should be expected in the first couple of days after firmware updates, but some users had reported decreased battery life more than 48 hours after updating.
Some theorized the problem was related to high refresh rates in iPhone models with ProMotion, but reports weren't isolated to those models. Others guessed it might have something to do with aging batteries. In any case, addressing the issue through an update confirms it was a software problem.
To update your iPhone or iPad, head to Settings > General > Software Update.