Update: well I could not reproduce what you are looking for so I went to the experts. I attended the Microsoft Ignite conference in Chicago and so was the OneNote team. So I decided to ask this question and see if they had an answer. They told me that the name of the notebook is only bold when changes to the book are made and when it should be synchronized. It is not bold when it is the active book you are working in. They said it should be an issue with the persons OneNote installation.

I know not the answer what you wanted to hear but that's all I can come up with. Sorry!

Maurice