Mine also started happening within recent weeks - I tried chatting with Apple support few days ago, and they insist that battery test is fine. My symptoms are exactly same - percentages jump inexplicably, and had few times phone shut off completely from as high as 70% remaining.
I did perform restore via iTunes and a computer as advised by Apple Support, and so far not seeing any improvement. This is very frustrating problem and I wish Apple would address it more seriously instead of ignoring so many of us having this issue.
I noticed that using Camera kills the battery much faster, or instantly. I had a case last week where Barcode Scanning App accessed the camera for few seconds, and that took battery down from 50% to 1% instantly. Most previous times when phone shut off into black screen was also when trying to take photos or videos.
Waiting to see if we can get some real support going for these issues here.