Well, this is actually a by design feature AFAIK (according to Sumirei from Nexon).
It's implemented to make fights it harder. So I'm changing this from a bug to a suggestion if you don't mind.
I can confirm that.
Certain monsters have a purposefully-added "NOHEALTHBAR" attribute. Ordinarily the health bar is visible by default; though this option was added on purpose to add to the challenge of the Dragon Trial.