This is certainly something new - or at least, not something of which I have seen previous reports.
I would initially suggest that you for force-close the Calendar App, then relaunch Calendar:
Quit and reopen an app on iPad - Apple Support
Force-closing the Calendar will force the App to restart, instead of suspending the App. This may be enough to correct the problem. If you continue to have difficulties, a forced-restart is the next step:
- 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 other iPad models: 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.