Volumetric Lightmaps Do Not Sample Correctly Under Landscapes

Per Licensee:

 I noticed that the Volumetric Lightmap does not build at correct density if an object is under the level of the terrain. Is this a feature or a bug? I'm creating a under-terrain dungeon and I need to have high density samples also under the terrain level.

1. Open attached project
2. In Viewport, Show > Visualize > Volumetric Lightmap

Result: All meshes placed above the landscape produce high density lighting samples within the Lightmass Importance Volumes surrounding them, but not if they are placed below a landscape

Expected: A higher density of samples will be produced around static geometry within a Lightmass Importance Volume even if it's below a landscape