Able to cast smite during Cassiopeia ult stun

[Gameplay] You are fully able to cast chilling smite (and potentially other targetable summoner spells) through the petrification of Cassiopeia's Ult

It happened twice while I was in game as Udyr. They both happened when we were already level 18 so I am unsure if level has any bearing on it for whatever reason.

1. Buy the jungle upgrade for chilling smite (blue smite)
2. Have an enemy Cassiopeia ult you while facing her so that you are stunned
3. Use smite on her or anything nearby

Expected Result: No cast will occur because you are stunned

Actual Result: Smite casts with no problems

I am not sure if this affects other summoners like ignite, exhaust, or even red smite. It is also worth noting that I could not use ghost while stunned so it may only be targetable summoners.