when using an item with the enchantment overlay (such as written_book or golden_apple with a damagevalue of 1, aka the notch_apple) as the advancement icon the enchantment overlay will show in the whole square in both the tab (if the advancement is the root advancement) as well as the toast while the advancement page is open.

If the advancement page is not open however, it doesn't display the overlay fully but only on the item the way it's supposed to be.