Sion Q getting interrupted during Banshee's Veil

I recently had a game where I had a Banshee's Veil on Sion, and while my Banshee's Veil was up I had my Q canceled by an enemy Twisted Fate's Gold Card, even though I wasn't stunned.

Steps to reproduce:

1. Buy Banshee's Veil
2. Charge Sion's Q, Decimating Smash, while Banshee's Veil's passive Spell Shield is active
3. Apply a stun effect to Sion (this probably works with other hard CC effects, but a stun is the easiest example)

Effect: Sion does not get stunned (as intended), and the channel on his Q gets canceled (which is likely not intended).