Hi
I answered a bit more in your other thread: RTS Unit can handle limit of navmesh - #2 by aron_granberg
The avoidance in the RTS beta has some additional tweaks however to make agents actually move around locked agents. This does at the moment not take into account the edge of the navmesh however, so they may just run into that instead and be unable to move past it.
I think this is just due to coincidence. The one side happened to be slightly closer to the target. Maybe you could order all agents to move towards the enemy + a very small random offset. I think that would make them pick sides a bit more randomly.
Looking at your video, that does not look like the rts demo though. In the rts demo the agents should avoid locked agents at least a bit better than that. Was the video taking when using the rts demo?
Below is a video of what I get when making agents try to move past locked agents