[GAMEPLAY] Fiora Q+W combo doesn't stun despite blocking a hard CC

I was Fiora against a Veigar. Veigar used his cage. I dashed into the cage with Q and immediately activated W. I got all visual feedback indicating that I executed the move properly: a/ the Riposte AoE became Yellow, b/ I got the white text saying "Spell blocked". Veigar was also clearly in the AoE (he tried to Flash out but both end points of the Flash were in the AoE). Despite everything, Veigar wasn't stunned.

Repro step:

1. Veigar uses his cage around Fiora.
2. Fiora Q+W on Veigar.
3. Veigar should be stunned but is not.

Repro-rate: unknown. Highlight+Replay: https://drive.google.com/drive/folders/0B0Jfeo3_M0vBSk1INWdaS2tkQVU?usp=sharing