So I had this issue for ages and couldn’t figure out what was wrong because my meshes weren’t lighting correctly even though I had created a secondary UV. from Epic told me how to fix it. Here’s what he said:
In the Mesh Viewer in UE4 make sure that you’re got the Destination Lightmap coordinate set to 1 instead of 0. This will be located in the details panel to the right.
Hopefully, that’s your issue.