I also have an issue where the NPC’s take a large unneeded detour. If the nav mesh was generated correctly.
On this screenshot you can see the patrol path suddenly going to the back instead of going forward. Eventhough the nav mesh is entirely green. There are no obstacles on the ground.
This is a screenshot from the same area in UEFN. You can see a faint blue and yellow line just following the path forward. It’s on terrain. These are two patrol path groups, one for an attacking army in one castle to the other, and one for an army in the other direction.
And even though the navmesh is completely green as seen on the first screenshot, and the patrol path is just following the straight line forward, they take a detour through the forest. 
In blue circles the nodes of the patrol groups and red the lines the NPC’s actually follow.
No obstacles on the path they should take between the two nodes, which is the shortest path going forward.