On MacOS Big Sur, messages only show in the all inboxes selection when they are unread. Both read and un-read messages messages appear in the individual inboxes. I didn't have this problem before. It might have come with my upgrade to Big Sur or possible Catalina. Is there an option to change this behavior?
I saw one post in the Apple forums on the problem. The poster resolved it by deleting an in box that he thought was causing problems.. I don't have any I can delete.