I think the "Replies" area should work like Inbox, where you have to click "Clear" to mark something as read, instead of it being marked as read as soon as you open the Replies tab. The trouble is, you can click on Replies by accident, or start reading some and then be taken away or click away for whatever reason. Then they all get marked as Read, and you have to realise and navigate back to the "Read" area, which is not going to happen every time. So it's too easy for replies to disappear without you realising. If you had to actively click "Clear" or "Read" instead of them being marked as read automatically, you would not miss any.