Quinn's "Harrier" sometimes doesn't proc.

I don't have any screenshots, but I'll post what essentially happens.

1. Wait for Harrier to cast
2. See the vulnerable icon on a champion or minion
3. Shoot champion/minion with basic attack
4. Attack only does basic attack damage, vulnerable icon is still there
5. Have to shoot a second time to proc it.

Something seems super bugged here. There are no shields or anything like that in play when this happens.