I had this issue earlier today on https://community.wanikani.com/ as well, at least on mobile. After visiting the new topic and navigating away, the title became grey as if it were read, but it still had the blue dot indicating it was unread.
@Stephen I don’t think I’ve ever seen this issue before, even when using the browser back button.
This one has been around a while, I am pretty sure @sam and @eviltrout are aware of it. Much more pronounced if you use the browser back button in my experience.