Gameplay - Potential Lissandra ult bug

I was playing in ARAM just now, during the game I ulted myself numerous times.

First I noticed the visual effect wasn't synced with the sound effect, so essentially I will hear myself ult, but only until later Lissandra will become untargetable.

In the last team fight, I ulted myself, however I died immediately after I ulted (for which I clearly saw myself becoming untargetable and in the ice form). It was clear to me that between my ult, and my death it was less than 2.5 seconds . So I believe there might be a bug considering Lissandra's ult and its invulnerability.

Anybody similar issue?