Now that there are replies on the thread, the issue on that thread not clearing Unread has vanished.
Taking this data and other examples in the past, it appears that when there is a "pending moderation" post in a thread the Unread state will not be cleared when reading that thread until/unless the "hidden" posts are revealed.
Sounds like a design bug in the forum software. Saying I have "unread" posts in a thread when that includes "posts I cannot read due to moderation" seems wrong.
Problem understood methinks. Feedback for the forum software providers at this point.
