When Import a meshes then open the staticmesh editor and turn on the "Generate Lightmap UVs" in the Build Settings. Then hit apply button to genereate the Lightmaps. It will generate the Lightmap but it will not change Lightmap Coordinate Index to the new index the new light maps is. So in the most common case the index is 0 but the newly generated lightmap will 1 but it stays on 0. It should automatically switch the index to what ever the new light map index is.
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There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-18800 in the post.