I should point out it was this way from about 50% onward, even as she was taking damage. I think she goes invuln when you have the kill the void goops and avoid the massive aoe, but the health bar never returned to the normal image even though I could kill her.
I encountered this bug today. I was confused for a while, because I thought they had changed her invulnerability phases. But yes, her health bar falsely showed as invulnerable from the first invulnerability phase until her death.