Adding some extra info, once the lightmap is built, changing between VSM and Cascade I see this difference in code paths:
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This difference, that being said, also happens in CPU Lightmass, which makes me think that actual ShadowMap in that channel ( channel 0 ) might be incorrectly generated in GPU lightmass and is simply not used when VSM are active as that bNonNaniteVirutalShadowMaps will return true, so INDEX_NONE will be used instead.