There is a degree of pathfinding I've found, they will generally dodge your troops a bit in open ground. Possibly the AI can't tell that the only way out the map to a certain point is across an easily defensible route, and so soldiers avoidance routines don't give them any options when there is only one way to go - across the bridge. And also the same with city gates.
So in other words this problem (which I've had) is probably a lack of programming for this sort of situation in particular in my ignorant opinion.
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