I’ve seen that before.

(Honestly, the web app is a little janky. Not sure about other apps.)

The trick I used to get it to stop indicating a message for anyone who might want to know:

• On the “inbox page” (/inbox in your instance), switch from “Unread” to “All”.
• (Get ready because there’s a timing component to the next three steps.)
• Switch back from “All” to “Unread.”
• The “Mark All As Read” button should appear at least momentarily while the page is loading.
• As soon as you can, start rapidly spamming the “Mark All As Read” button and cross your fingers.
• If you’re lucky, that did the trick and it won’t display a “1” on your bell icon. If not, repeat from step one. It shouldn’t take much more than three or four tries to make it work.