Trust me, I’ve double checked everything that is of my current knowledge. I’ve also tried revising the mesh and cleaning it up… removing doubles, deleting loose geometry, rebuild light-map pack etc. but it was already optimized and cleaned up. I also tried enabling “Generate Lightmap UVs” to see if Unreal could handle it but to no avail.
It’s really strange, there doesn’t seem to be any artifacts with lighting (black triangles / splotches) and the error coloring doesn’t show anything either.
I think later I will delete the mesh entirely and reimport from scratch and see if that fixes it. Until then I have it set to ‘movable’ to rid that warning.
I’ve included an image. I believe the UVs are never outside the 0-1 space when exclusively letting blender build a lightmap for you. I don’t usually triangulate a mesh before import, I just did that with this mesh after the errors for extra precaution. The UV channel “00” may have mirrored/overlapping UVs but that’s the texture channel, the LM channel, to my knowledge, has none of those things.
Make sure that you’ve got your lightmap coordinate index set to 1. So far, you’ve shown that you’ve set the build settings (for auto-generated lightmap UVs) and that you’re looking at UV 1 for the lightmap UV.
Under the static mesh settings tab you’ll find Lightmap Coordinate Index. This needs to be set to 1 so that it uses the correct UV. The build settings you showed are only used for generation.
There was a bug in 4.13.0 that would allow this to happen. I think this was fixed in 4.13.1 or it may be 4.14, I can’t remember exactly.
You’re not the only one who has fallen victim to this Build Setting. Most who don’t actively use or worry about a lot of the lightmapping intricacies or have custom lightmaps really know or worry about this, so definitely not anything to be embarrassed about.