In the latest snapshot (24w33a) that was released recently there was a bug in the enchanting table. When you try to enchant a part of your armor in the enchanting table it doesn't work, which shouldn't happen because in the latest version (1.21.1) you can enchant a part of your armor without problems in the enchanting table. Regarding the tools, they work correctly.
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is duplicated by 3
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Can confirm that it affects both vanilla items and custom items with the 'enchantable' component (in the goat horn picture's case, I even added enchantable tags to the goat horn in a data pack).
It seems like armor is missing the enchantable component entirely, as removing the component from the item does not reduce the component count. Adding the component manually through commands, e.g.
/give @s diamond_helmet[minecraft:enchantable={value:1}]seems to allow the armor to be enchanted through the enchanting table again.
Can confirm: