A notification that just wouldn't go away

[HeroLinik]

Spoiler: show

This picture pretty much explains my problem.

So basically, if you can't see the picture for whatever reason (even though it should appear) what I have here is that I had gotten a notification that says I have a PM from Kley. I opened the PM, and left the window, but the notification just wouldn't go away. I've refreshed the page, logged out and back in again, but it still wouldn't go away, considering that according to the image, I've read all the PMs so far, and the notification pinged today. In fact, I had to physically mark it as read because it wouldn't go away on its own.

Why wouldn't this notification go away?

[Enjl]

just click the checkmark which appears when you hover over the notification, or hit "mark all read"