Piglins do not use the correct texture for Diamond, Iron, Chainmail and Golden and Leather helmets when using Programmer Art resource pack. Instead, they use the default helmet textures.
I have attached screenshots comparing each helmet on both a Piglin and Zombie on the Programmer Art texture pack.
I'm not sure if this also affects Netherite helmet as I couldn't tell the difference there.
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Duplicate of MC-169889. Works as intended.