The only workaround that I have found to work 100% of the time is:
- Open the email completely (double click to open it in a new window).
- Use the 'Read / Unread' button at the top.
- Close the email window.
Every other solution people have proposed has not worked, or if it did, 5% of the time it would partially work. Some emails would remain read, while others would revert back to unread. This new version is completely broken, in my opinion.
This is such a cumbersome workaround for anyone that gets a fair amount of emails each day. It should be a priority one issue for the devs, but I guess not.