[GAMEPLAY] Lulu's Whimsy (W) bugged on enemies that can become untargetable

This happened in my latest ARAM game. Basically what happened was that I was playing Lulu and there was an enemy Yi. I would try to cast Whimsy on him as I see him charging our team. However, if he used Alpha Strike (the ability where he becomes untargetable, not positive on name) then it wouldn't work. Makes sense because he just went untargetable. However, Whimsy would then be on cooldown even though it didn't work.

It would be difficult for me to replicate this because I would need someone's help. I'm also not certain on the timing. Basically, you need to try to cast whimsy on the enemy Yi just right before he goes into alpha strike. In that case, obviously he wasn't polymorphed, but W will still be on cooldown. I'm not even sure this is a bug because I know that some abilities like Wukong's ult persist through Whimsy, but just thought I should mention it because its really frustrating.