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Do you use the built in Mail app, and which account is misbehaving (iCloud, Outlook, Gmail, ...)? Is your iPad up to date (iOS 12.1.4]?
Force close the app: slide upwards from the bottom so that you can see the App Switcher, then slide the thumbnail towards the top of the screen.
Turn off Mail for the account, restart your iPad, and enable Mail once more.
Take a look at the account online. Open www.icloud.com via browser. Are the deleted emails there as well?
Thanks for all your help.