Leather, Iron, Gold, and Diamond Horse Armor have translation keys in format of item.horsearmor<material>.name meanwhile the Copper and Netherite Horse Armor have translation keys item.copper_horse_armor.name and item.netherite_horse_armor.name respectively
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We're tracking this issue at MCPE-225693, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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