Using the App Switcher, begin by force-closing the Mail App...
To do so, swipe-upwards from the bottom edge to the centre of the screen - and briefly pause before lifting your finger from the screen. This will open the App Switcher. Identify the Mail App window from the visible thumbnail images, swiping to the left or right as necessary. Having identified the Mail App, swipe the thumbnail upwards to force-close the App.
Next, perform a forced-restart of your iPad:
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.

After the iPad restarts, open the Mail App as normal - and refresh the account mailboxes. Hopefully this will have cleared the problem.