iPhone not updating its battery status Apple's looking into it
According to a page added to Apple’s support site last week, Apple is investigating reports that the battery status indicator on the iPhone 6s and 6s Plus stops working correctly when you manually change the time on your phone or change time zones. The problem only occurs on the iPhone 6s and 6s Plus, according to Apple, and only happens when you manually change the time or time zone setting.
Apple’s looking into a more permanent fix, but in the meantime, if you run across this issue, Apple says to you restart your iPhone, then set your phone to update its clock automatically. To do so, open the Settings app, tap General, then tap Date & Time. Next, slide the “Set Automatically” toggle to the “on” position—the slider will turn green.
Doing show should keep this bug at bay, but if the issue persists, Apple recommends that you contact its support team.