"Activity Stream" still shows me read threads?

[Beetlecat]

This may be more of an Invision functionality than a KSP implementation one, but I'm trying to streamline my forum catch-up times, and trying to use a custom activity stream to show only unread topics in forums I follow. It works great until it doesn't:

Old, previously read topics are constantly re-appearing in the stream, even though I have selected "unread." I can re-visit all of the all of the old threads, and it seems to catch up/remember that I've seen them, then eventually the first page of topics will all be old, *already-read* threads again. Is this a cache/cookie-ish kind of thing? I'm not blocking the forum in any way I know of, and every other read status works fine in the normal forum views.

Anyone else successfully use the activity streams to view unread topics like that?