Quicksilver slash bug

Hi all, i'm just wondering how is this possible or if it's a bug. Zed ults on me, after 1 or 2 seconds i activated my qss but it didn't remove the debuff and the mark triggered on me anyway. Am i missing something (notice zed's QSS removes my spears outplaying me xD )? Here's the video. https://youtu.be/VGSgpGNkmJc
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