I did this fight pretty much every day for the first month it came out, and I never had any problems. Recently I've been doing it again to get a few random items for twinks, and I have not noticed any change with 3.3. However I don't stand in the middle. I stand over by the door directly between two fresh horses, with my main a bit out, and they do indeed focus the main character from proximity to start. They only go kind of random after I have to switch horses on the main. When you are standing right next to fresh mounts to switch to, it doesn't matter if the first incoming drops one of your guys fast. I highly recommend setting up by the door directly next to the fresh horses, as opposed to doing it in the middle, especially if you are having trouble.