Actually it's more of a blizzard problem making it not work.
Currently everyone is using the "Random Que" instead of single queing like before. So the bug is all the random people are not getting placed in a que with the specific BG ques.
It's on MMO-Champ today with news about it.
Your's may be working but that is simply due to people single queing with you. Most servers are overwhelmed with Random BG ques and not singles. I know all 6 servers I raid on have the same issue.
This will be easily fixed once we get a random bg option.
Do you have a log? I have not tried Mixed at all in 131 so I apologize if I'm not aware of any bugs with mixed. If you can post a log of what happens during the mixed-av when you start it, things might move along faster.
It will come we just need patience and understanding that this stuff is complex to release due to keeping things safe and secure from the warden monster