may be related to #161138 if i set the view to show only unread articles, then i press '-' to navigate to an unread article, akregator opens an unread article, the article gets marked as read, disappears, then the next article is opened, marked as read, disappears, then the next article, then the next, ... until the entire view/feed is marked as read. i did not notice this behaviour with kde4.
fixed in 5.6.x or 5.7.0
*** This bug has been marked as a duplicate of bug 350731 ***