The First Empress Cygnus Defeatd in KMS

Those x4 HP bars you brought up are EXACTLY why I came up with that theory.

When Wizet first programmed boss HP bars, they probably didn't anticipate giving a boss (Or at least the part of the boss that gets the HP bar.) 2 billion+ HP, so they probably didn't design the bars to hold that much HP. And they might have not wanted to add "x10" for some reason. (It is a pretty stupid system. Especially when different bosses have the same number of bars but not even close to the same amount of HP.)

Of course, I wouldn't rule out the possibility of actually having her heal herself to artificially extend her HP if the game's programming won't allow an individual mob to have more than 2100000000 HP.

The HP bars work fine, it's just empress specifically that is broken. Hilla worked just fine with the exact same HP as empress before they fixed her in KMS. The only bosses that have multiple HP bars are bosses with multiple forms, such as zak. One bar = one form of the boss. Note how each form of zak has differing amounts of HP, yet the bars still drain correctly.

There is nothing wrong with HP bars or their display. There is something wrong with Empress's healing skill. Something causes it to spam at an insane rate with no cooldown after she hits a certain threshold of HP.