Notifications count does not display the real number when the events are marked as read if there are more than 21

STEPS TO REPRODUCE

Preconditions: An user (e.g. U1) is already created.

1. Login as U1
2. Go to Notifications > Settings
3. Set all Pages Notifications to ON
4. Login as Admin
5. Create/edit/comment some pages (app. 30 actions)
6. Login as U1
7. In the notification list, click on the 'check' icon for some events (e.g. for 3 events) to mark them as read
8. Observe the unread notifications count after every 'mark as read'
9. Refresh the page

EXPECTED RESULTS

Due to the fact that the number of notifications is much higher than 21 (close to 30), when some (few) events are marked as read, the notification count should display '20+' after every 'mark as read' action and also after refresh.

ACTUAL RESULTS

After every 'mark as read' action, the count decreases: '20+' > '20' > '19' > '18' ....and so on, but after refresh, it displays back '20+', so the decrease does not reflect reality.

A solution might be hiding the counter when notifications are marked as read if the number of events are higher than 21.