Currently, all messages received from MAM are marked as READ. With XEP-0333 it would be better to mark them as read, only if:

• message was sent (by us) after received messages
• message was confirmed as read by us (XEP-0333)

With that in place, we would have "in-sync" unread messages between clients so that we would know that message received on another client but not marked as read is not read - currently we mark it as read as it was in the MAM.