\data\guardvillagers\loot_tables\entities

guard_armor.json

\data\guardvillagers\loot_tables\entities\armor_sets

armor.json

Datapack not working for new version despite matching the new 1.21 datapack format. Instead, all the guards have lost held items entirely.

Try using this for the guard_armor.json instead
https://pastebin.com/krFzcmsY
You seem to have accidentally put two entries for the offhand slot.