Any time you have a topic open on your screen, its status is set to “read”. If you want it to have the status “unread”, the topic needs to be not displayed on your screen.
What happens when you press Shift+u on a topic is that it sets the topic as “unread” and redirects you on the home page.
If you scroll the topic list to the topic you just unread, you’ll see it doesn’t have the lighter color of the “read” status.
Before
After
But I agree that the behavior feels strange. A quick popup mentioning we have unread a topic could be a good addition.
If they’re new, they’re unread. 