Normally the UE4 takes the first uv channel as the lightmap layout, but sometimes those uv’s are overlapping. This will cause some problems (black spots on your mesh, entire black mesh,…). Sometimes you dont have to create the lightmap, but just when the first channel follows the rules of a lightmap (no overlapping uvs). This happens with all programs, not just blender ^^ Here you can get more information about lightmaps: