Thanks for the info. The check mark actually doesn't relate to read or unread status, it just allows you to select a message to then take an action on it (like delete it or mark it as read or unread). After you mark all messages as read, their subject lines should not appear in bold. Can you try these actions and see what happens?
1. Navigate to your main PM page
2. Check the box next to your first PM
3. From the drop down menu, then click "Mark Unread" - the subject line should be in bold.
4. The message will still be checked.
5. Now click "Mark Read" - the subject line should change from bold to not bold.
When you click Mark Read for that first message, is the title still showing up as bold?
It works just as you described, at least in my experience. Thanks for the clarification.