I am trying to understand what to do in this task.

Should notifications vanish from the page if their state doesn't correspond with the filter?

@Mooeypoo according to the specs in {T132738: Mark groups of notifications as read}, the answer is 'yes'.
That brings the question about symmetry in the behavior for the 'Read' filter: with 'Read' filter, marking notifications as 'unread' the behavior should be the same as in

a) get marked as read and b) disappear.

Change 298910 had a related patch set uploaded (by Mooeypoo):
Relate read-state filter and mark read/unread action

https://gerrit.wikimedia.org/r/298910

Change 298910 merged by jenkins-bot:
Relate read-state filter and mark read/unread action

https://gerrit.wikimedia.org/r/298910

Checked in betalabs - the correct behavior is in place.

• when a notificaitons (or a group of notifications) is Marked as Read it will disappear from the filtered view, e.g.. click 'Unread' on Special:Notifications -> 'Mark as read' a group of Notifications -> the group disappears.

• when a notification(s) marked as Unread, they also disappear from the filtered view, e.g. click 'Read' on Special:Notifications -> click on the empty dot to mark a notification(s) as Unread again -> the notification(s) disappear.