Yasuo ultimate and Blitzcrank rocket grab

I was playing against an Yasuo top and a Blitz support. We had a Braum in our team, and 1 time blitz grabed Braum with his Q and Yasuo was able to ult Braum, although Blitz's Q is not a knock, but a stun. Blitzcank didnt hit his E (that is a knockup), i saw that Yasuo used his ultimate in the middle of blitz grab. Is that a bug, or it's normal?