Therein lies the problem. The issue isn't with syncing read status once you open mail.app - that happens, if sometimes not instantaneously. The issue we're describing is that it doesn't sync read status until you open mail.app, and thus notifications/badges don't disappear. Other mail accounts don't exhibit the same (broken) behaviour.
Let's say I have a mac, and iphone and an ipad. I get 2 emails at the same time, one from icloud and one via an exchange account. I read them both on my mac. You'd expect them to be marked read instantly on all devices. Both the iphone and the ipad will display a red (1) until mail.app is opened because icloud mail is wonky.