This mission has a bug regarding AI pathfinding which is known by the devs, and has been known for at 2 yrs now.
In my playthrough of this mission the bug seemed to only affect 4 waves headed from the southern most road. The first two waves got stuck in one of the ambush patches along the road, and the other two got stuck along the river bank behind trees that I didn’t find until after I had scouts on the opposite side of the river.
Seeing how in both cases the occurrence of the bug specifically happened along the southern road, the only solution I can think of, until it is patched, is to deforest the southern road up to your wall.