Unless fully shut-down, your iPad will still be running background tasks while locked and not being actively used. While battery utilisation will largely depend upon your iPad settings and installed Apps, you might reasonably anticipate seeing 10-15% battery depletion for each 24hr period of inactivity - more if some Apps are performing background updates.
Rechargable batteries are consumable items - and for every recharge will lose capacity. This degradation is cumulative; an iPad battery is rated to retain ~80% of its initial capacity (when new) after 1000 recharge cycles - and as such will lose ~1% of its initial capacity for each 50 full recharge cycles (one full cycle being any combination of charging that together sums to 100%).
The behaviour that you observe is characteristic of an aging iPad battery. If you are concerned, you might be best advised to directly contact Apple Support and request a remote battery diagnostic test. You can contact the Support Team using the Support link at top-right or bottom-left of this page.
Alternatively, you can initiate and manage your support cases from Apple devices using the excellent Apple Support App. If not already installed, the App can be downloaded from the App Store:
https://apps.apple.com/app/apple-support/id1130498044
You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to hands-on test and assess your iPad:
Genius Bar Reservation and Apple Support Options - Apple
