Apple has acknowledged an issue on its latest iPhones that may be resulting in inaccurate battery percentages being displayed for some users.
The issue appears to be related to time, or time zone, changes on the handsets. Apple's Support site states: "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."
For now, if you encounter the issue, Apple is advising that you restart your iPhone, and then go to Settings > General > Date & Time to ensure that 'Set Automatically' is turned on. Apple says it is still "investigating the cause and a solution" to the bug.
However, if you've been experiencing the same issue without changing the time or time zone on your iPhone, Apple advises that you contact its support team for further assistance.