The raid boss bar solely updates when one of the raiders sustains damage. It remains unchanged if a raider is healed by potions of the regeneration effect, a situation commonly facilitated by witches. However, if you damage a raider after healing them, the boss bar updates, potentially increasing due to the augmented total health.
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We're actually already tracking this issue in MC-263799, so this ticket is being resolved and linked as a duplicate.
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