After the Grindstone removes all enchantments from an item, it does not reset the prior work cost for the same item on an anvil. Items that are too expensive to work will still be too expensive after the grindstone has removed all the enchantments.

This behaviour is probably unintentional. It's easy to work around this behaviour by combining the item with another item with a lower work cost.

To replicate:

(Creative mode)

Enchant two swords with six enchantments using enchanted books from the creative menu: Sharpness V, Looting III, Unbreaking III, Sweeping Edge III, Knockback II, Fire Aspect II.
Get another two unenchanted swords.

(Survival Mode)

Place the enchanted sword on an anvil -> It will be "Too expensive!"
Remove the enchantments with the grindstone.
Place this cleaned sword on an anvil -> It will be "Too expensive!"
Place this sword in the TOP slot of the grindstone with another clean sword in the bottom slot. Take the repaired sword.
Place this repaired sword on an anvil -> It will be "Too expensive!"

Place the enchanted sword on an anvil -> It will be "Too expensive!"
Remove the enchantments with the grindstone.
Place this cleaned sword on an anvil -> It will be "Too expensive!"
Place this sword in the BOTTOM slot of the grindstone with another clean sword in the top slot. Take the repaired sword.
Place this repaired sword on an anvil -> It will be enchantable.

Repairing items in the crafting table clears the repair cost, so this is the expected behaviour.