Audacious Charge and First Auto don't always apply challenge

I'm not sure how to record clips, I'll try to work around to that soon. I main Xin and I've seen this happen several times in games. Audacious charge is supposed to apply challenge as well as any subsequent auto attacks, however when I audacious charge to an enemy and auto, my ult still knocks that same enemy away. I'm not sure what's causing it, but I'll keep trying to reproduce it consistently.

I know Xin isn't a high priority champ, but this bug is a huge issue as it's causing him to knock away targets he set out to prioritize without the means to reach them thereafter or greatly delaying his knockup.

Pls. Send Halp Rito.

Love,