When you attack a llama, mule, donkey, or horse, you will see a heart that says that you have done a lot of damage at first.
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duplicates 1
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This issue is present for mules, donkeys, and horses too.

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We're actually already tracking this issue in MC-195931, so this ticket is being resolved and linked as a duplicate.
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It appears the llama's health is higher than the max health, and when taking damage, it caps the health properly, making it seem like a lot of damage was dealt.