These past few days, the notification badge is persistent. Even though there doesn't seem to be any unread message, the badge won't go away. The browser says it's three while downloaded app says it's two. I cleared cache and signed out. For a moment, the notification disappears on downloaded app, but it comes back quickly.
@Hakoishi You probably have more than one Microsoft account. If you only have one email address you may have a personal and a work account that both use the same email address. It may be easiest to use the web-based version of Teams in an "in private" session, log in with the personal account, check messages, then repeat with the work account.