When you summon a wither with a custom name it does not display that name above the health bar... instead it just simply says "Wither" I have a few more issues now pertaining to using a wither for a custom boss heath bar but I need to investigate them further before bringing them up... This one though is bugging me a lot.. If you are going to take custom names out of the equation when it comes to health bars, then allow us to assign health bars to a regular entity such as a skeleton or witch... I am assuming though for now that this is in fact a bug because I doubt the minecraft team's goal is to hinder our creative nature...