The issue is seen when using a normal navmesh inside of a World Partitioned level. Adding a Nav Modifier Volume to a Level Instance, and placing the Level Instance in the world will affect the navmesh. However, if you move or reposition the Level Instance, the navmesh is not updated to reflect the current position of the Nav Modifier Volume. If you add a static mesh that affects navigation to the Level Instance, the navmesh will update around the static mesh as the Level Instance is moved, but it still does not update where the Nav Modifier Volume is placed.
Expected Result: The nav modifier volume causes the navmesh to dirty and rebuild as it is moved.
Actual Result: The Level Instance moves the nav modifier volume, but the volume does not affect the navmesh.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-197023 in the post.