https://bugs.kde.org/show_bug.cgi?id=350731 describes a problem where, if the search filter is set to "Unread messages" and the "Mark as read after..." is set to zero or a short time, as soon as an article is removed from the list the next article is selected and then marked unread in turn. The effect of this is, with these filter settings, that all unread articles in a feed are marked read as soon as the feed is shown. These settings worked and had the intended effect in KDE4.
The diff here is the patch as attached to the bug comment #39, with the API doc cleaned up a bit.