Expectations: The samples should only surround the lightmass importance volumes.
Imagine if we build a 8K*8K map with lightmap importance volume at the corners of the map, even using stream levels(because VLM can not be streamed now) the build data only for VLM would be around 10M*8*8*8=5120M！which even only contain the lowest level of VLM
This issue is similar to this one other than using two small Lightmap importance volume instead of a big one
There's a huge mount of samples filled in to a huge cubic bounds that encompasses the two lightmass importance volumes
Check the output log see LogVolumetricLightmapImport: Indirection Texture 128x128x128 = 8.0Mb
Save the map, the build data is about 10M
Show those samples in viewport like this
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-76613 in the post.