Different protection enchantments are no longer compatible with each other

The bug

As of this pre-release, you can no longer combine different protection enchantments. In the changelog, there is this line:

Reverted enchanting system to how it was before 1.14

... but MC-141961 has been resolved as "Works As Intended", thus making it unclear whether the change in 1.14.3-pre2 was supposed to make protection enchantments incompatible with each other again.

To reproduce

1. Grab a piece of armor, and two enchanted books with two different protection enchantments (for example: Protection IV and Fire Protection IV)
2. Combine the armor piece with one of the books on an anvil
3. Try to combine the armor piece with the other book
   → It isn't possible to combine the armor piece with that book.