With the recent enchanting changes, unbreaking cannot be applied to items with the Unbreakable tag, but mending can.

To reproduce:
1. Go into survival mode
2. Run 

/give @s diamond_sword{Unbreakable:1b}

3. Attempt to apply an unbreaking enchanted book to this item via anvil
It is correctly prevented.
4. Attempt the same with a mending enchanted book
It is incorrectly applied.

Given that the item effectively has no durability, it should consistently not be able to receive mending if unbreaking is prevented as well.