You read right amigos. Using Space Bar to preview photos in the Photos app is no more. Why Apple decided to do this I will never understand. So we tweak our workflow to adjust to these changes just like anything. There are two ways to open a photo in the Photos app, the first way is to hit the “Return” key while the photo is selected or double-click the photo you want to open and you get the same result. I personally prefer double clicking. And then to return back to the album, just hit the “Esc” key.
Some have suggested ways to bring back the Space Bar way of doing this by creating an app shortcut in preferences. Read this thread to find out more about this: https://discussions.apple.com/thread/8554200