First, check the model without a material. IF you have a hard edge on a fully rough black material, the issue is the model. Near nothing you do will change it.
Second,
You definitely need to add all the correct (clean) maps.
Roughness and Spectacular have a tremendous effect on the seam areas.
Normal maps more obviously so.
You take the normal map (which you have none of in the screenshot) and plug it into the Derive TangentBasis Node. This can be a neutral normal map as well, it just corrects the result.
For Skin you set the material to
Surface - Masked - SubSufrace Profile (and assign a proper one).
Make sure Tangent space normal is checked - should be the default.
Tick Use with: Skeletal mesh, Static Lighting, Morph Targets (and you said alembic? if so Geometry cache).
In the material graph, connect all the proper maps, and as mentioned, derive the Tangent like this
Additionally on the mesh Build settings you need to make sure that Recompute Tangents is set - should be default.
This is NOT the same as checking it within the material under LOD. There is no need to check “recompute tangent” on the skeletal mesh LOD material in .25 or .26. It works as is.
IF you still have issues the fault is either with the textures, or with your model.
Possible solutions are the same as for any seam. Bumping up UV precision (use full or change threshold UV/tangent, and or tick High Precision Tangent.
For Hair I still have Recompute Tangent checked in .26.
BTW, light settings DO NOT matter, but mesh settings do.
on .26 I have Skin Cache Usage specifically set to enabled for all the modular meshes.
Further the Lighting options are a bit more complex - works in tandem with a directional light and is not necessarily going to work for everything.
The directional light:
Oh, and yes in .25.3 transmission was bugged. in .26 it seems to be working fine.