From GameWatcher: “Warhammer: Vermintide 2’s Bile Trolls are some of the most challenging foes the Ubersreik Four have to face. They’re capable of both strong melee attacks and spitting corrosive bile at anyone unlucky enough to be in front of them, so it’s easy to see why players started panicking when the creatures seemed to also be able to fully regenerate. Thankfully, that’s just a visual health bar bug added in the recent Sister of the Thorn update.”

Source: N4G PC Warhammer: Vermintide 2's Sister of the Thorn Update Introduces Bile Troll Health Bar Bug