Boss HP bars have disappeared

I have no idea how to help, but have you tried reinstalling? That's always the first thing that comes to mind when a problem arises.

I doubt it's a problem with his client since Zakum, Pianus, and Pap show up find. It's problem just something Nexon decided to get rid of. I don't boss often so you'll have to wait for someone else to confirm that the weak bosses lose the HP Bar.