How to reproduce: create a creative world, hold a Knockback II book, attack a mob and watch it fly.
I expected that the book's enchantment would not be applied to the action.

This bug has been around since long before 0.12.3

It appears that this bug is limited to sharpness, smite, bane of arthropods, knockback, fire aspect, looting and silk touch, but not all enchantment options have been attempted.

While this bug appears innocent and WAI, it can be easily exploited by the player. Some examples:

• a sharpness V book can be used as an unbreakable sword
• a fire aspect book means you never need to cook meat
• a silk touch book can collect all blocks not destroyed when mined by hand
• a looting book can be used for the final blow to kill a mob and get more loot
  Books with multiple strong enchantments compound this exploit quickly.

[MCPE Mod] AMAN- A duplicate of this issue has been resolved (MCPE-9886) so this is probably fixed for a future release. I can not confirm yet.

Tested on:

• Samsung Galaxy S4 Android 5.0.1 0.12.3 single player survival
• Asus B121 Tablet Win 10 Pro MCPE 0.12.3 single player survival