Hi J, that sounds annoying. Are you using the Outlook desktop app or the web version?
If it’s the desktop app, the draft might be stuck in your mail sync. Try deleting the draft while Outlook is online, close the app, then reopen it after a few minutes to let it resync. If it still comes back, sign in to Outlook on the web, delete the same draft there, then clear your browser cache. This usually removes drafts saved on the server and stops them from reappearing.