Steps to reproduce:
1. Apply the "Thorns" enchantment (any level, but I specifically experienced the bug with Thorns II) to any piece of armor (I had it on an iron chestplate)
2. Let a skeleton attack you
Unfortunately, I was not able to reproduce this consistently.
Care to explain how this "works as intended?" It doesn't make sense to me for it to be intentional, considering that skeletons attack using a projectile...