[GAMEPLAY] Mordekaiser's Lich Bane passive doesn't proc on small monsters

I was playing a custom game as Morde myself against Bots, and bought myself a Lich Bane. Lich Bane's passive is said to proc bonus magic damage on the next AA after casting a spell, but what happen was that the bonus damage doesn't proc on small monsters.

1. Buy Lich Bane on Mordekaiser.
2. Cast an ability (mostly his Q)
3. Hit the first two AAs on small monsters.
4. Bonus damage doesn't proc at all.

Whereas, I tried hitting the first AA on a large monster instead, it does proc off the Lich Bane passive.

Is this a bug or somehow intended?