It seems that the light of the enchantment is not displayed correctly when changing the Piglin's head equipment to the equipment enchanted by the command.
Command:
/replaceitem entity @e[type=piglin] armor.head minecraft:golden_helmet{Unbreakable:1b,Enchantments:[{id:"minecraft:protection",lvl:5s}]}
Thank you for your report!
We're actually already tracking this issue in MC-173747 and MC-172643, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main tickets it would be appreciated.
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