I typically use the “Unread - Oldest” filter-sorter setting. For some feeds (it looks like maybe only reddit top month feed), it continues to show some (not all) read articles. The view shows them as read, but they don’t get filtered out. Most articles don’t come back after I mark them as read, but a few do. I see this same behavior on newsblur.com on all the OS/browser combinations I’ve tried (Windows Firefox & Chrome and Mac OS Firefox). Here’s a screenshot taken after selecting a feed that had no unread items when I clicked on it.