There is 1 unread email notification but all of the emails have been read

My Inbox notified that I have one email to be read. I do not see any unread email in my inbox. The email is nowhere to be found.
Already tried this:

1. Sign in to your Microsoft account.
2. Navigate to the Inbox page.
3. Open your Inbox folder.
4. Just below the +New button, there will be a "View:" category, click on that then select Unread. (Note: Doing so, all of your unread emails will be populated.)

No email in this list.

Very disturbing this...

Any idea?

Tib