Nav mesh volumes generate bounds around actors based on the actors volumes. When blueprints are used to combine meshes, the overall volume of the blueprint is used rather than the respective individual volumes. This can be seen by the forest blueprint developed in the tin man level.
By enabling the "Draw Path Colliding Geometry" in the Project Settings > Nav Mesh the volume bounds of actors become visible to help visualize how actors affect the nav mesh
As shown above the grey volume encompasses the entire blueprint and obstructs the nav mesh. When the blueprint is removed, the nav mesh can cover the entire space as expected.
Matt Young developed a yellow brick road geometry and textures that were integrated into the scene by using splines. The spline generates the geometry along the spline path and adapts to the path as the spline is edited.The spline is generated on the same surface created by UE4 that is used to place and paint foliage. Both the foliage and the splines inherit the scaling proportions of the surface. Neither the foliage or the spline seems to affect the nav mesh's generation.
After noticing the how the blueprints forests volume in the TinMan area, I spoke with the team they were confused because the nav mesh worked with them. Slightly adjusting the Z axis will either block the nav mesh entirely or allow the nav mesh to generate but still produces a gapbecause of the end of the blueprints volume.