the damage command won't work on a mob that is already taking damage.
reproduction:
1) spawn a mob
2) damage it
3) use the damage command very quickly to deal damage
4) watch as it does not work
Comments 3
Because that's how “iframe” works. I don't think it's appropriate if /damage command ignores i-frames.
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The whole point of invulnerability time is that the entity cannot be damaged in any way. Asking the damage command to override that behavior is a feature request, the fact it doesn't is not a bug. It applies damage like any other method.