I was under the impression that every hit on mount gives you a 20 second Heavy debuff, no matter your DR and it resets the 20 seconds every time you get hit on mount.

Since this patch, it has been changed. Not sure if it's a bug or intentional, as right now being hit on mount gives you the standard Heavy duration (not 20 seconds) and will resist after 3 times.

Pushback isn't even working, see the video below. The first clip is right when the match has started so there is no possible way this person has got hit on mount or off mount.

https://www.youtube.com/watch?v=d8E8m6Z6-_4