Marked as read

I had a bit of catch up to do on the forum, so I wanted to use the unread post (1). The forums that I wasn't interested in, I marked all as read (2). Now colour me surprised when I pressed the unread post link (1) and saw post marked as read, but un the unread list (3)

Is it a bug or did I just expect a different result?

(1)

(2)

(3)

I just tried to reproduce that but for me, the thread from that forum that I marked as read disappeared from the page as it's supposed to. Strange.

But this problem kind of feels familiar. I think it might have been brought up before at some point? I'm really not sure