Unable to mark as read Forms app notify

Hi Friends,
I’m using Linux Ubuntu 20.04, where I’ve installed NC client 3.1.1.
All is working fine, except for Forms notify.

In fact, some weeks ago I built a simple form, where I asked information of various kinds to the users.
Now the form is closed, but I stil receive the response notifications and seems impossible to mark them as read.

Where I’m wrong?

Many thanks!