That's a bug in the BBS software, we think. Hopefully it'll go away when we upgrade to the next version.

In the meantime, to clear the stuck-on unread-mark, do this:
1. Make sure you're done reading all the messages on the BBS you wanna read.
2. Select "MY HOME" from the bar up top.
3. Select "MARK ALL READ" from the bar at the bottom near your user icon.
4. Select "MARK ALL POSTS AS READ" from the confirmation screen.