Untargetable champions like Vladimir vs Karthus ultimate bug?

Hi friends, I was playing Vladimir on ARAM vs a Karthus and I noticed that whenever I went into W, Karthus R would still target me. I've never seen this interaction or bug until now. Before, this never happened to me in my previous ARAM games against a Karthus when I got the trade or reroll for Vladimir. Is this a bug or is this intended? Thank you. I will link a video from a YouTuber by the name of Eldimarix.