My Mail app (MacOS Catalina 10.15.3) displays phantom unread messages
Quit Mail if it’s open.
In ~/Library/Mail/V7/MailData, delete any file that begins with “Envelope Index,” such as Envelope Index or Envelope Index-shm.
Your home Library folder is hidden by default. To display it, choose Finder > “Go to Folder” and then enter “~/Library.”
Open Mail.
Mail creates new Envelope Index files. This process may take a few minutes, depending on how many messages Mail is reindexing.


It will fix your issue.