When opening emails on my iPad, they are not being marked as read

This is an issue sometimes seen within this Support Community - and is one that I have experienced myself on a couple of occasions.

The resolution is relatively straightforward - requiring that your initially force-close the Mail App - then force-restart your iPad. Before doing so, if you have not already done so, ensure that you have updated your iPad to iPadOS 13.6 - as this may itself clear the issue:

Settings > General > Software Update

To force-close your Mail App, you’ll need to summon the Task Switcher; swipe-upwards from the bottom edge of the screen, pausing briefly before lifting your finger from the screen (if you have an iPad with a physical Home button, a double click will also summon the Task Switcher). Identify the Mail App among the open Apps - and simple swipe the App off the top of the screen.

Having now force-closed the Mail App, you can now force-restart your iPad:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On an iPad with Face ID: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

Hopefully this will have resolved your issue.