Empty Volumetric Lightmap

Development Rendering
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Hello,
I am in 5.5 working on a baked-lighting project using the regular Build menu (not GPU Lightmass) and have come up against a weird issue. My volumetric lightmap seems to be completely empty. When I visualize it in the viewer I get no samples at all and the fog/movable actors within the interior space light as if they are fully exposed to the sun/sky. I have a LM Importance Volume in the level and it is correctly sized around the area. I even tried adding LM density volume set to 0,0 to see if I could force more detail but nada. And yes, I have built the lighting numerous times for iteration and testing.

Is this a known issue or just something I have overlooked?

Thanks for any help you can offer!

UPDATE:
I did check the output log after running a build and found these lines that seem related. However, I don’t know what would be considered expected sizes/counts and so don’t know how this would compare.

LogVolumetricLightmapImport: Imported Volumetric Lightmap in 0.014s
LogVolumetricLightmapImport:      Indirection Texture 16x16x16 = 0.0Mb
LogVolumetricLightmapImport:      BrickData (all levels) 0.6Mb
LogVolumetricLightmapImport:      Bricks at depth
LogVolumetricLightmapImport:          0: 0.6% covering 100.0% of volume
LogVolumetricLightmapImport:          1: 3.7% covering 9.4% of volume
LogVolumetricLightmapImport:          2: 95.7% covering 3.8% of volume
LogVolumetricLightmapImport:      Bricks in each level
LogVolumetricLightmapImport:          Scifi_Lab_Blockout              164 bricks      100.0% (Persistent Level)
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UPDATE 2:
Some further testing:

  • Swarm Agent logs show that it is finding the LMIV and adjusting the area and samples accordingly
  • Halving the VLM detail cell size in World Settings shows an increase in brick counts/sizes in the logs
  • Testing with GPU Lightmass instead of CPU does display the samples as I would expect and movable actors begin lighting more appropriately although the resulting bake is much messier

So it seems like it is finding the volume and accounting for it but the resulting volumetric lightmap is not loading or not being used in the level afterwards when CPULM is involved.