An armor stand which has armor enchanted with thorns damages the player when he hits (tries to break) the armor stand. This can result in a hilarious death message.
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wobst.michael
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I'm not sure whether that's fully intended as it didn't do it in 1.12.2 suggesting that it is either not supposed to do it at all or it should always have but it didn't. Also, the death message is always going to be correct as the lang files have a specific format for this.
What else did you expect. You've hit thorn enchanted armor therefor the attacker takes damage. Also the death message looks right in that case.