The "title-changed" signal is not emitted if the document doesn't have a title (because <title> is missing in a HTML file or becuase the file cannot have a title, like images), so GtkLauncher and midori both show the title of the previously visited page.
Considering that we are going to add a title property I think it makes sense to set it to "" as soon as a new load starts, thus emitting "title-changed" (and "notify::title").
Issue was fixed with a signals rework couple releases ago. I think bug can be closed.
The "title-changed" signal is deprecated. If you use "notify::title" you can rely on having a title set even if the title is empty.