Vi ult bug?

I've been playing alot of Vi since Riot buffed her ult so she can no longer be interrupted while in the ult.

Here is the issue, when I have been playing Vi against champions such as Syndra and/or Azir. They can still push me away in my Ult, but the slam still connects. However this does not appear to happen with other knockbacks such as Alistar's and/or Tristana's Knockbacks.

So to summaries, Vi's Ult against Syndra and/or Azir will be normal until they try and push me away, Vi's ult will still be affective but Vi will be slamming them in the ground from a larger distance away.

This is really annoying for me when Vi is one of my main champions, the game feels really unfair in a match-up against Syndra or Azir, they get to do damage to me and knock me away when I'm supposed to be on top of them.