Sivir / Zilean Bug

I was playing Sivir bot against a Zilean Support and noticed a spellshield bug: Zilean threw 2 Bombs at me. Altough he landed the fist one** i blocked the second one with my shield**. I didnt get stunned or something but the bomb i still had on me became invisible ... but detonated shortly after. This happend about 3 times. Its not a gamebreaking bug but still a bit confusing.