i havent tried the apple 12w charger yet, but am using those anker chargers. have 0 problem.
i think based on the maximum normal usb chargers can provide 5v 2.4a ==> 12w is actually the max ipad can take without those fastcharge technology....
just try another charger, and see how it goes.
depends on what you are doing on your ipad, the power draw *may* be larger than the power supply can provide, therefore the device will draw power from battery.
this also happened to some macbooks as well, it's not unusual.
let your ipad finish initial setup/download/index/optimize, let it sit there for a while and close some app, then do some test.