Urgot vs Ornn interaction

I was playing a ranked game as Ornn against Urgot and had the puzzling experience that Urgot's E (dash-displacement-stun)still worked while I was in Bellow's breath (W) which is supposed to make Ornn *Unstoppable* no displacement spell, no stun should work on him. But it did and it promptly led to me feeding my buttocks off and to a really hard game. Is this a bug or is it an intended interaction? If so, I am curious about the mechanics behind it.
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