Keep mark an unread email as read

My issue started yesterday after upgrading to IOS14 on an Iphone 11, since the upgrade I have a notification of 1 unread email that doesnt exist, already logged to Icloud.com on laptop and there are no unread emails. I also did the “mark all emails as read” and nothing. The email app keeps reporting the 1 unread on/off, appears then syncs and it goes away and a minute later comes back again the notification and back and forth.

Make a new Smart Mailbox that only includes Unread Mails, do they show in that Smartbox?

If you have no POP accounts & they’re all IMAP accounts then in Mail’s Mailbox menu choose Rebuild.

Any luck with that?