[Gameplay] Spellshielding Siphoning Strike

So I noticed this while playing as everyone's favorite spook, Nocturne, the other day. Nasus went to use his Q, Siphoning Strike, on me (the particle effect went off, showing his next basic attack would proc the ability), I activated Nocturne's W, Shroud of Darkness, and I took full damage.

At first, I thought I might have just failed to activate the spellshield in time. However, time after time, Siphoning Strike went right through the Shroud of Darkness. Then I thought that maybe it just wasn't meant to work (for some crazy reason, even though I had been able to shield against other empowered attacks like Twisted Fate's "Pick a Card" and Garen's "Decisive Strike")...but then I had a look at the 5.22 Patch Notes and found that Nasus' Siphoning Strike was among those listed that should do nothing if blocked.

So this is a bug, right? Am I right? Or am I going crazy over common knowledge of something I'm just too nooby to understand ("Oh, Siphoning Strike can't be Spellshielded. EVERYONE knows that hahaha!")??