IPad unread message discrepancy

The unread message count can, on rare occasion, be incorrect.

Force-close the offending App - and then perform a force-restart of your iPad:

Quit and reopen an app on iPad - Apple Support

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:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. 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.

I’ve seen this happen. Just ignore it and move on.