In the Test Messages forum, Peggy's "Say Hello!" thread was turned into a redirect before I got a chance to read the new messages. As a result the link was a redirect to another thread, but the redirect was marked unread which also caused the forum to become unread constantly even though there was no thread to actually read.
Clicking the "Mark this forum read" link fixed it thankfully so I didn't have to be stuck with the false unread image.
Perhaps, although to an extent it's still treated like a real thread since I can mark the forum read, the thread gets marked read, and when I go back to the forum home it finally appears as it should. So somewhere there's something still treating the redirect like a thread when it shouldn't be.