Poor title, difficult to describe.
In the attached screenshot, I’ve selected “All Site Stories” and am showing “Unread” items. Items from Daring Fireball specifically (and for my account it is happening primarily with DF, but that may be coincidental) are showing as unread, back to March 4.
But in the feed itself (about 2/3rds down the left-hand side) it only identifies 5 items as being unread, and when I read just that feed item it behaves correctly.
If I view it at the folder level (in my case I have DF in a folder named “Usual Suspects”) I see the same behaviour as when viewing All Site Stories.