Disenchanting w/ Grindstone does not remove high XP cost for later repairs

Using the grindstone- I disenchanted an Iron Pickaxe that had Efficiency III and Unbreaking III- and then went to repair the now normal Iron Pickaxe, but the Anvil still required 31 XP to repair this item to full strength. Tried it again with a simpler Efficiency II Iron Pickaxe and that too required the same amount of XP to repair regardless of it's enchantment status being removed. It's also worth noting you can continuously place an Item in the grindstone left slots and play the grindstone noise by selecting the output item- despite no actual changes being made.