IPad Pro 12.9 battery loses charge

My iPad Pro (1st Gen w/ iOS 11.2.6) discharges down to about 37% and then suddenly shuts down. I plug in the cord and it reboots, showing 2% battery. However, once it’s fully booted and I sign on, the battery indicator in the top right corner of the desktop header again shows 37%. If I disconnect the charger cable while it’s still showing 37%, the iPad shuts down again. If I charge it up further then unplug, it shuts down again once it discharges to 37%.