I deleted a feed with no unread posts and then re-added it. The feed’s posts remained “read”, which is questionable behavior but reasonable.
However, I have my feeds set to display unread posts only. This means that my new subscription simply results in a blank white window, which appears broken.
My suggestion is to show something (ie, “All messages already read. Click here to view read messages.”) when there are no unread messages, so that the user knows that everything is working correctly.