Why can't I mark threads as read any more?

In one of my Loomio groups, we have a practice of closing threads once a decision has been made. Whenever someone closes a thread, it comes back into my Inbox as Unread.

There used to be a quick way for me to mark these threads as 'read' without having to open them and close them again. I think it recently changed to a tick, but now it has disappeared altogether.

Is there a reason for this?