I do apologize if this is not a bug or an intentional change.

When at an Anvil, it is not currently possible to combine two 'Protection IV' Enchanted Books of the same level (or higher) to increase the level of the Enchanted Book.

• 'Protection IV' Enchanted Book + 'Protection IV' Enchanted Book = X (N/A)

• 'Protection IV' Enchanted Book + 'Protection V' Enchanted Book = X (N/A)

• 'Protection IV' Diamond Boots + 'Protection IV' Diamond Boots = 'Protection IV' Diamond Boots (No increase to the Enchantment Level)

• 'Protection IV' Diamond Boots + 'Protection IV' Enchanted Book = X (N/A)

• 'Protection IV' Diamond Boots + 'Protection V' Enchanted Book = X (N/A)

I have tested combing other types of Enchanted Books at an Anvil to see if they will result in same issue or if they will produce an Enchanted Book of a higher level.

• 'Efficiency IV' Enchanted Book + 'Efficiency IV' Enchanted Book = 'Efficiency V' Enchanted Book

• 'Efficiency 'IV' Pickaxe + 'Efficiency IV' Enchanted Book = 'Efficiency V' Pickaxe

• 'Sharpness IV' Enchanted Book + 'Sharpness IV' Enchanted Book = 'Sharpness V' Enchanted Book

I unfortunately don't have that many Enchanted Books to test, the only Enchantment I can find that seems to be effected is 'Protection'. The other Enchantments I have been able to test seem to function as expected.

Many Thanks,

Stay safe