Screen burn-in can happen to iPad screens ( and iPhones and practically any other phone or tablet ) over time and with excessive screen use.
I have had this happen a few times on two of my iPads at diferent times and you are not going to like the solution, but it works!
To fix or mitigate the screen image burn-in, charge up your iPad to 100%.
Completely power the iPad OFF and DO NOT turn it on and use it for, at least, 3 days, minimum!
4-5 days would be better, but 3 days may clear this issue up.
One or two days of being powered down and non-use of the iPad is NOT enough time to have the screen image burn-in disappear. Have tried to reduce the time down. Doesn’t work!
DO NOT USE this iPad for a minimum of 3 days!
DO NOT even think about turning the iPad on for 3 full days!
Let the LCD pixels “de-energise” themselves for a time and let the screen calm down for a spell.
After, at least 3 days, power up your iPad and start using it normally as see if the screen image burn-in has disappeared.
Your iPad should be fine for quite awhile, but once this screen image burn-in happens once, it can return again after some prolonged time has passed.
No way to know when the screen image burn-in will re-occur.
If it happens again, repeat the procedure I outlined here.
If you shortcut this process and power up the iPad after a day or two and the screen burn-in is still present, you will only end up starting this process over from the beginning and will have to wait out another full 3-days!
You will just drag out this process further.
Leave the iPad powered completely down for, at least, a full 3 days before trying a power up and use of your iPad.
The idea is to let everything inside of the iPad simply cool/calm down and not get anymore pwer, especially the screen, for a few days!
Best of Luck to You!