Easily replicable - mark any two messages in a sub unread, click on another sub, back to the first. A blank message panel will show. Press space and it'll jump through the first unread, marking it read, and land on the second, displaying it and marking it read.
Middle-mouse-button for next unread works correctly, landing on the first unread and staying there.
I think this is a bug from a few days ago, rather than with today's build 333.