I think the pathfinding isn't made for finding paths outside of the layout. If it was, that means that a sprite could "choose" to go from A to B in a maze by running around it ^^
That said, you have 3 solutions.
First, you can make your layout bigger, letting them spawn inside the layout. You just have to set the scroll to the center of your level, so mobs spawn can't be seen and it looks like they walk inside the layout.
Second, you can, after a mob spawns, make him walk inside the layout first and then look for a path to the destination.
Third and last, you can change the spawning point. I just moved the "origin" point of your spawning zone and it works nicely right now : capx