This happens to me – it's nothing to do with ignore lists, as I have an empty ignore list.
I think it might be due to a forum setting, but it's a puzzle, as the forum still knows which posts are read and which aren't (because it still colours the unread posts' icon to differently.) If you're on the right page the "View First Unread" link still works.
It only happens on threads I haven't read for a few weeks – either because they haven't been active, or I just haven't got round to reading them...
It is very, very annoying.
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