imabubble, sadly I believe you are experiencing the symptom of having lost one or more cells. Whereabouts are you located, as maybe a forum member closeby with CaniOn could help verify this? Suggest you get it to a dealer while you can still drive it there. The good news is that the battery is still covered by warranty.Edit:
imabubble wrote:...Just today, I noticed the car wasn't charging, even though the green "ready light" was on, so I unplugged and replugged it in...
Perhaps a minor correction to what you wrote: in order for the "Ready" light to come on, the car must be "Started" using the "ignition key". The vehicle's interlocks will prevent that from happening if the J1772 connector is plugged in, even if it is no longer charging. Conversely, if the car is in "Ready" and you plug in the J1772 connector, it will immediately turn off the car.
I'm pretty sure you didn't intend to say that the "Ready" light was on, as the basic issue I believe you have is that the car stops charging (and now, doesn't even charge) despite the fuel gauge showing only a partially-charged battery.
Please do keep us apprised of your developments.