iPad Air 5th Gen battery deteriorating by itself.

I usually let my iPad drop down to 40 percent before putting it down to charge, last few weeks whenever I closed my iPad and folded the Smart Folio Keyboard and let it sit overnight, I saw my charge drop from 80 percent to 0. When I looked at the battery usage history it said that the screen was on when it should have been covered with the folio. My auto-lock time is 15 minutes and the Lock/Unlock feature is on for the folio.

Yesterday I tried leaving the folio open, not covering the screen, and letting my iPad sit in flight mode. I saw that the charge hadn't moved. What could be the reason that my iPad loses charge by itself, I am concerned that this would affect my battery health percentage.