Anvil ignores item in 2nd slot

Put the summary of the bug you're having here

What I expected to happen was...:
Enchantments of first and second item (diamond sword) should add together

What actually happened was...:
Enchantments of second item were ignored. Tried multiple swords.

Steps to Reproduce:
1. Put sword in first slot
2. Put sword in second slot
3. Anvil did not have red X, so should function

4. Put cursor on result sword, no new enchantment from second item