Fiora and Vi ult bug

Playing jungle Vi; everything going as usual. Then I go to gank Fiora in the top lane. I land my vaultbreaker for the stun/knockup and immediately follow it up with my ultimate. While Fiora is in my ult ( she is currently at the very top of my slam when the "bug" happens) she ults, using it to break out of mine. I don't play a lot of Fiora, and only know what her abilities are so I look up her ult to see if it is capable of breaking CC but didn't find anything.

Edit: She had no QSS