Okay first I would update IE9 as explained why here
Next I would test to see if it is a browser related issue by trying an alternative browser such as chrome/firefox etc... The bug may only occur on IE9 and not chrome/firefox. If the bug really annoys you and it occurs only on IE9 you could use another browser for browsing the forum until they fix the issue.
I'd also think it's worth creating a thread on that forum to see if other users are experiencing the same issue and to alert the staff of your problem because like you said the forum got upgraded recently. It sounds like a forum related issue, a bug that needs fixing.
I'm afraid there is nothing we can do, everything indicates that it is a forum bug.