Apply the attached data pack to a world, and open the beacon UI. Then attempt to place a netherite ingot inside the payment slot. As expected, the slot forbids placing it inside. Then add a copper ingot, and note that it's accepted as a valid payment. This is to be expected, as the data pack replaces netherite with copper as a valid payment item. Despite this, the item sprites shown to the left indicating valid payment items remain the same - the UI falsely infers that netherite is still a valid item to use in beacons, and omits copper.
Expected behaviour would be for only items which can be used in beacons to display here.
Just thinking out loud, how should this be solved if you add a dousen items to the tag? Having a button in the gui that'll show valid items, like a recipe book?
Can confirm.