I found the issue, and I can reproduce it in a blank project.
https://dl.dropboxusercontent.com/u/3332789/asdfgaseds.jpg
Just do this, set default pathfinding to pass through friendlies.
https://dl.dropboxusercontent.com/u/3332789/dsgasd.jpg
This is why. The AI sees that tile as visible even though there’s no tile it can actually attack it from
You can also reproduce this with player units, they’ll run to index 0 as well, but properly path there atleast.