[GAMEPLAY, VISUAL] Sad Robot Amumu was not shown as "transparent" in bush in Howling Abyss

the sad robot Amumu skin was unlocked for me in an ARAM game and I found Amumu does not go like "transparent" when he stepped in to the bush. He looks like as he was not hidden from enemy's vision.

How to reproduce:

1. use the howling abyss map
2. choose the Amumu and the sad robot Amumu skin
3. go into a bush

I only tried it once only because I do not own the skin. but I stepped into the bush for multiple times in that game and the same thing happens.

In the attached screenshot, Amumu is in the red circle and we can see that the Olaf in front of Amumu is shown as "transparent" but Amumu's green head is still clearly visible.