Can't mark message unread while reading message

With the latest update 6.601, when I click on a message and read it, there’s an open envelope that you can click on to mark the message unread. This is still the case with the latest version but when you go back to the message list, the message is still marked as read. I highly depend on this working as it’s the only way I can get back to a message I want to read later.

The icon in the toolbar depicts the action that will be performed when you tap it. An open envelope means the message will be marked as read. A closed envelope means the message will be marked as unread. (The behavior was the same in previous versions).

I know how it works. Funny, right after using the new update, it didn’t work. But now it appears to work fine. It may have been some kind of glitch after the initial update.