Shadow artifacts away from UV shell edge

Hi, I’m new to UE4 and just working my way through getting a nice clean lightmap & set of lighting UVs on my furniture models here in a test scene. I’ve been getting black artifacts and i thought they were just the result of too many small UV shells and seeing the black pixels pulled in at the shell edges so I started combining more of the smaller UV shells and that helped but here I have an issue that is not at the edge of a UV shell and I don’t know what I am seeing.
In shadow the mesh shows black jaggies like the rough edge of a UV shell in the lightmap but it isn’t at the edge of the shell and it goes away in full illumination. I have screen shots here of the area under a dynamic shadow; when lit there’s no artifact; when the shadow passes across the area you see jaggies. I also have a grab here of the UVs on the front face of the armrest showing how the hard edges arent at the edge of the shell. They are, however, at a hard normal edge on the mesh - could this be part of the issue? Smoothing group/hard normal artifacts??


I looks like there’s a hard edge or smoothing group split there, that’s why UE4 is adding a seam. If there is a hard edge or smoothing group split, you need to split the UVs as well.

There is a hard edge there in the mesh. So where’s UE4 putting the seam? in the lightmap? The UVs are still one shell.

Smoothing out those hard edges did remove that seam as you suggested. I still get these faceted looking artifacts though - any clues to these? All the normals here are smooth