While working on multibounce in 4.18 I compared vs 's implementation and found the results to be nearly identical (screenshots attached). However, 4.18 also has a filtered cubemap representation for the skylight which causes significant differences, which are expected. I think the difference you are showing is actually from the filtered skylight.
You must set bUseFilteredCubemapForSkylight=false in BaseLightmass.ini to compare 4.18 multibounce to 's to get apples to apples.