The notification number displayed in the top right corner won’t disappear.

Normally, I would click on the notification post, jump to the thread, check the post, and the notification would clear.
I would repeat that for each notification.

However, even after going back to the homepage and clicking the notification again,
or logging out and logging back in, the notification still wouldn't clear.

Eventually, it disappeared after I clicked the "Mark all as read" button.