Apple released iOS 11.4 to the public at the end of May and since then, a decent number of iPhone users have been complaining that the update is draining their battery.
Most users seem to be complaining that when their phone is in standby mode, the battery will still drain as if the device was in use, according to 9to5Mac.
So far, it’s undetermined what is causing this issue, but it seems associated with the iOS 11.4 update. Now that iOS 11.4.1 has released, it will hopefully solve the problem.
In Apple’s support forums, it seems like most of these users that are complaining are using iPhone 6s or SE. Some users are reporting this battery drain happening on other phones and iPads, but the most common device in the thread is the 6s family of phones.