But it sounds like that is the expected behavior. You are focused on that project, and promoting a child to the same level as the parent would not be possible since it would have to cancel the focus. I think that what you want is a feature request.
You may request a feature that make it so that where when a project is focused and you want to promote a child above that project, TaskPaper automatically focuses on the parent project. It actually sounds like a pretty good feature request to me!