'Overkill' can be achieved without 'Enchanter', but does not register

I just noticed that if a sword was anviled together with an enchanted book (from a dungeon), it could be used with a critical hit to deal enough damage to a mob to achieve Overkill, without getting Enchanter first.

What I expected to happen was...:
When I hit a mob with the sword and dealt enough damage i would get the achievement Overkill.

What actually happened was...:
Nothing.

Steps to Reproduce:
1. Find a sharpness IV enchanted book.
2. Use an anvil do add it to a sword.
3. Kill a mob with that sword and get a critical hit.
4. Make sure you did at least nine hearts of damage.