Have just tested this and it works fine for me, from his mystery/normal attacks/chase. Just to be sure you know that it's personal passive like water mains Neurotoxin, so it only works when Haku does a critical and not whole team.

http://i.imgur.com/d4Qq0N8.jpg

Weird, he works for me so it's seems to be something on your end, i asked another person to test with me as well. Maybe you just upgraded him and need to clear cache and restart game or it's something server related. I am on S17 see if you can ask people on your server if their Haku works to know if it's just you.