hmmm, there is this note about lighting in the docs:
"Each individual mesh instance has its own shadow and/or light map generated by Lightmass, and these are tiled together for each precomputed batch. There are several settings on the Static Mesh that should be checked for precomputed lighting to operate well with instanced foliage. Lightmass is less forgiving when generating shadow maps for instanced meshes, and incorrect settings could lead to black meshes after rebuilding lighting. "
But I would have thought that would affect all LODs and also your 4.27 version. Out of interest though (I’m building support for Impostors into a plugin so want to learn as much about all aspects) - what happens if you increase the lightmap size?