Okay.. so I THINK I can explain your problem, but I don't know the solution off the top of my head.
In my examples, the pathfinding is not working how you want.
In your example, the pathfinding IS WORKING how you want.
However, collisions are not. I've seen people have this problem with bullet behaviour. I'll explain:
Let's say your solid object is 8 pixels wide. let's say a bullet heads toward that object with a speed of 20 px/tic..
This can result in the bullet jumping entirely over the solid, so no collision is actually registered.
I believe something similar is happening to your pathfinders. Maybe when they turn or something, they become close enough to the other side of the solid, that they end up there.
Honestly, I'm not completely positive on that.. (I could be completely wrong) but seeing as you don't seem to getting much help (rare around here), I figured I'd share my theory.. Hope it helps!