Is it a faulty battery issue or a firmware/software issue?

My iPad mini 4 shuts down after a few minutes of usage and shows a low battery icon even if the battery percentage shown was 100%. It’s been having this problem since last year so I tried updating the firmware to iOS 14 to see if it fixed the issue but it didn’t. Coconut battery even showed that the battery seemed normal (though I am not sure if it is reliable).

However, I noticed that if I played a video for few minutes while charging and then unplug it and let it continue playing, the iPad doesn’t shut down. I can then use an iPad without a problem as long as I don’t plug the charger back or shut it down. Does anyone else have the same problem?