[GAMEPLAY] Anivia's Ultimate Disappearing

Played a game with Anivia on Summoner's Rift. I noticed that several times my Ultimate's CD was acting funny, and after closer observation I saw that my ult in fact was sometimes disappearing completely even when I was still in range. After that I paid special attention to it and saw it vanish twice, but it seems random. am currently playing more games with Anivia to see if it can be reproduced