I’ve been having this problem since I started using UE4 about one year ago, but never cared to search for answers until now, which has become a bigger problem.
So, when I import objects from 3ds max and bake the lights in UE4, everything acts as normal, but if I have to go back to make adjustments and reimport the asset with different UVs or even sometimes it’s a completely different object substituting the original, what happens is that the lightmaps for these objects get all crazy. Usually making weird patterns, as if the whole lightmap is tiled thousands of times.
My workaround for this issue is erasing all the lightmaps for the level (I turn on “Force no Precomputed Lights”), restart the editor, import these assets with a different name, and then use the Replace References tool, so they go in place.
This process gets tiresome and for larger levels is a problem because of the light baking times.
I should enforce that this isn’t a problem with the UVs, they are on the correct channel, and they are made correctly. This is a bug or something else I’m not seeing.