Apple has acknowledged an issue iPhone 6s and iPhone 6s Plus owners have been experiencing. The problem is when the power percentage reading locks up and freezes, which doesn't aid the owner when attempting to determine just how much power remains inside the battery. While Apple is currently investigating the cause, it's believed the issue has something to do with altering time zones.
As noted in the support document:
"If you change the time on your iPhone 6s or iPhone 6s Plus manually or change time zones when traveling, you might notice that the battery percentage doesn't update."
Luckily, the temporary fix is fairly straightforward. Simply reboot your phone and check to see that date and time are configured automatically by heading to Settings > General > Date & Time. However, if you continue to encounter this issue and are not travelling between countries in different time zones, Apple urges you to contact customer support for further assistance.
Have you had an issues with the battery monitor on your iPhone 6s? Let us know in the comments!