Khazix stays invisible while amumu's using ultimate, is it a bug?

Hello admins and fellow summoners,

thank you for reading my post. I want to ask/report if its a bug this situation. When Khazix goes invisible and Amumu uses his ultimate ability on him, while Khazix is in range, Khazix stays invisible. It's funny because when amumu uses his Q ability on khazix (while invisible) he actually becomes visible. Am I the only one who witness this situation?

Thank you for your respond :)