Since it's working for me, and for you when you type it directly into the URL search bar of your browser, the only other thing I can think of is the <Album Artist> tag. If it is empty, MB assumes the <Album Artist> is the same as <Artist>. For the album you're searching is your <Album Artist> tag empty or populated? If it's empty, you may have found a bug, where <Artist> isn't being supplied to the search in lieu of <Album Artist>.