Outlook items previously marked as read get marked as unread

I'm using Enterprise Vault 10 with Exchange 2007 and Outlook 2010. I have a Public Folder Archive setup. Items deposited into this archive folder when archived get marked as unread. However when someone goes back and markes the items as read it lasts for about a day or two and the items get marked as unread again. Does anyone know why read items would continuely get marked as unread? I'm assuming it's something to do with Enterprise Vault since regular Outlook items in the user's inbox's do not have this problem.