How to get rid of invisible notification?

Question

Sometimes I have the problem, the I get a notification from Sololearn App like

Person X likes your comment
Person Y replied to your comment

However, when I open the app, these notifications vanish automatically and when tapping on the Bell-symbol, I only see the one from person X.
That's a pity, since I would also like to see the others guy answer.
Also, when restarting the app, there is always a red circle on the bell, indicating a new notification. But there is no new one.
What can I do?

Answer 1

That's what I did at first, but it still shows the red circle everytime I start the app.

Edit: After starting my app now, there is no notification anymore. Seems like a new, displayed, notification was needed, to fix this.
But I still cannot see the message of Person Y :/

Edit2: It's popping up again. Mark all as read does not help

Answer 2

Ok, as marking as read did not help, even after trying this multiple times and the red circle does not show up only after new messages (but pops again after again new massages), my solution is this:

Updateting the app :'D

Should have done this earlier...
Now I can also see the mentioned "lost/invisible" notification of Person Y :)