Suffering from the iPhone 6s and iPhone 6s Plus stuck battery percentage indicator? Apple has a fix—and by "fix," we mean "workaround."
Manually changing time zones can freeze your iPhone's battery indicator
Here's the problem: You're traveling and you manually change time zones on your iPhone. After that, your battery percentage indicator stops updating.
fix workaround: Turn off your iPhone by pressing and holding the power button until you see the on-screen slider telling you to swipe to shut down. Wait for the shutdown process to complete then press the power button again to turn on your iPhone. Now go to Settings > General > Date & Time and turn on Set Automatically.
That'll get your battery percentage indicator working again, but it doesn't fix the underlying problem. Apple says it's working on sorting out what's behind the issue and plans to release a bug fix at some point. Until then, it looks like letting your iPhone auto-switch time zones is the best option.