Final question: does your phone last "normally", e.g. does it still last as long as it used to before the bug started appearing?
If "yes", then it's a simple software issue with the battery calibration that doesn't affect the actual battery charge levels. You could try upgrading to Windows 10 Mobile, but I'd suggest waiting for the next update a few weeks after new year.
There's a stranger variant to that issue reported several months ago (almost two years back), where changing the region of the phone changed the battery charge level, i.e. from phone region A to region B the battery charge changes from 100% to 75%, but going back to region A from region B returns the charge back to ~100%. It's a variant because if you try to charge your phone in region B, it will "jump" from 75% to 100%.