I have observed your comments and like to share with you that NaN is returned because the width of shape is set to 2 by default so PowerPoint does not save any specific border for the shape.
LineFormat.Width = NotDefined;
whereas the width of the border, from the presentation shared by me (see ShapeWidthModified.ppt), is returned as 2 just because I have set it as LineFormat.Width = 2. However, I have logged a ticket with ID SLIDESNET-37028 to further investigate this and check if our product team approves the reason that I have just explained to you, or if it is really an issue. I will keep you posted about the findings of our product team here in this thread.