Have you performed a Factory Restore?
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
For diagnostic purposes, when you complete the Factory Restore, do not restore your data/settings from your backup. Instead you should initially set-up the iPad “as new” with your AppleID - and test the iPad. If the problem clears, the issue lies in your iPad settings; if the problem remains, then you have an issue with the iPad with which you will require assistance from Apple Support.
You can contact the Support Team using the Get Support link at top-right of this page. Alternatively, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:
https://apps.apple.com/gb/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 test and assess your iPad:
Genius Bar Reservation and Apple Support Options - Apple