Perhaps you have been using a CPU/GPU intensive App, or have perhaps been using the iPad with high screen brightness - any of which can draw more power than usual from the battery. Another possibility is the battery calibration - which sometimes requires a full charge to 100% to recalibrate.
Two immediate things to try:
1) Perform a forced restart of your iPad - as this can clear a stuck or malfunctioning App-process:
Force restart iPad (models with Face ID or with Touch ID in the top button)
If your iPad doesn’t have the Home button, do the following:
- Press and quickly release the volume button nearest to the top button.
- Press and quickly release the volume button farthest from the top button.
- Press and hold the top button.
- When the Apple logo appears, release the top button.

2) Temporarily change your iPad's charging settings to allow a full recharge cycle.
You should note that Apple's quoted battery performance relates to "normal" screen brightness (~30-40% this being adequate for most indoor ambient lighting conditions) with non-intensive Apps. Gaming and video streaming are particularly heavy on battery utilisation.