iPad battery calibration?
As resolved in comments:


Try calibrating the battery by fully charging to 100%, and then draining it to when it crashes. From personal experience, letting it die out slowly overnight is better than sudden crashes due to increased demand while using.  Some users report doing the charge-discharge cycle twice. https://apple.stackexchange.com/a/15568/313842
If battery is still out of order, do a hard reset. This is an advice on several blogs I saw, and now this answer is one of them.


  Press and hold the restart button combination. If the iPhone can't be shut down normally, simultaneously press and hold the right button combination for your iPhone:
  
  
  iPhone 6S and earlier, iPad, and iPod Touch: Power button + home button
  iPhone 7 and 7 Plus: Power button + volume-down button
  iPhone 8, 8+, and X: Volume up, then volume down, then power button
  
  
  ... Continue holding the buttons. Do so for 10 seconds until you see the startup Apple logo on your screen, then release the buttons.


https://www.wikihow.com/Hard-Reset-an-iPhone

Adding some advice: keep iPhone plugged in, or at least on higher charge levels. Same reason for which Apple advices the same while upgrading iOS