It appears that the Lightmass Volumetric Lightmap does not work with Landscape textures. When it samples the Landscape with some textured material, the resulting lighting build data does not match the actual texturing of the landscape at all.
This issue was reported and tested in 4.21.2 (CL-4753647). This was reproduced in 4.20.3 (CL-4369336), 4.22 (CL-4958363), and Main 4.23 (CL-4982248)
Results: Lightmass Volumetric Lightmap indirect light from the landscape is wrong, not matching the texture on the landscape surface.
Expected: Lightmass Volumetric Lightmap indirect light from the landscape is correct, having the same result as indirect light from the static mesh plane of same position, size and material.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-70009 in the post.