Why does my Facebook messenger show a notification when I have read all of them?

[AC Question]

why does my facebook messenger show a notification when I have read all of them.

I have cleared the data and cache . I have restarted my phone all to no avail

 

[belodion]

why does my facebook messenger show a notification when I have read all of t...

If it comes to that, why does my Tapatalk do the same?
I assume it's some glitch or bug.
Perhaps someone else here can enlighten us both.