If multiplying a mesh with ComplexCollision by inverse scale, the geometry may not be created on the mesh. This may depend on the assets of the mesh. In the sample example, the navmesh is not created when inverse scale is applied. If using SimpleCollision, the navmesh will be created even if applying inverse scale.
The splines of the SplineMeshActor are multiplied by inverse scale. In this case, not creating navimesh. It is expected that a navmesh will be created on the SplineMeshActor.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-160327 in the post.