Image retention is not (normally) an issue associated with LCD screen technologies. While most iPad screens have an edge-lit screen backlight, any dimming will affect the entire screen - and not just around the edges. If your iPad has software issue, a forced-restart may clear the problem:
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.

Force restart iPad (models with the Home button)
Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

If instead your iPad has a screen hardware fault, the only remedy will be repair/replacement of the screen - requiring access to either an Apple Store or Apple Authorised Service Provider (AASP):
Genius Bar Reservation and Apple Support Options - Apple