ue5.0.3 still has the same issue. The workarounds (even editing the engine source code) mentioned here don’t work. 
What I found out is that the seams get worse the more different the UV between materials is, but even with very similar density you never get it perfect.
The seams are driven by the normals.
Normals are chopped off if you chop it off, so you have to preserve vertex normals when you cut.
Generally I prefer cutting 1 ring ahead (obviously near impossible if triangulated) of the seam, slightly shrinking the last loop by some .05mm and extruding the vertex/merging at center to create the stump/ensure the geometry is manifold.
This usually leaves the normal data on the junction alone, so seams arent visible.
Milege varies, but it generally works across anything without needing further workarounds.
Obviously how the mesh gets lit has a drastic impact on what you see or don’t see where the seam is.
Also, mesh precision comes into play when you animate.
In theory both sets of verts on a juncture will have the exact same values.
In practice, to be sure of that the meshes cannot be positioned manually and have to all snap to the same pivot. And even with that you can still get situations where there is slight divergence.
Lastly, 90% of all situations where seams won’t go away with a good model, are the result of outdated or bad skin texture values/maps.
Roughness and specularity, as well as SubSurfaceProfile matter.
On SSP, the engine is still 100% interpreting map values wrong when using the character skin shader. Epic truly dgaf even if you need this to work correctly in production for both games and movies. Its been the same for over 3 years now.
With ue5 and lumen, there is a chance they changed the light so it now doesnt give you chicken pox skin at certain angles. A very thin chance unless the USF was changed since it scaled textures values incorrectly…
I would suggest you test with adjacent engine cubes to see if the seam persists or goes.
The default engine skm is a rectangle. Same diff.
Playing with this it seems like it’s not working very well, My color channels are not ebing respected 
After reading a few threads the solution that worked for me was turning on “Support Compute Skin Cache” and setting “Default Skin Cache Behavior” to “Exclusive” on the project settings. After that go to your skeletal mesh, on LOD 0 under sections, choose the model sections affected by the morphs and change “Recompute Tangents” to All.
This will fix both the normals and the seams seen below.
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So should just using a normal map to drive a a direct tangent output btw. And its probably cheaper.
Either way, the engine hasn’t improved any of the skeletal mesh skinning/texturing/lighting at all in spite of receiving tons of updates to all sorts of things which should have corrected the original issue.
They even added quaternions and other dedicated skeletal mesh stuff, but making sure morphs, composite characters, and shadows can all coexist in harmony… well, that is apparently too much to ask for still…
So taking over the rendering (away from the engine by using a tangent output) is likely far more beneficial in the end…
I did all this and I can’t solve my problems. I’m almost crying because I can’t solve this, I’ve been looking for solutions for 3 days and I haven’t been successful.
Like i said. Epic dgaf.
Try using the character normal as the direct tangent output for the skin material.
The matwrial bit is shown up above. You have all the relevant settings apparently toggled, but you either neglected the material snapshot or to acrually change it so you take over computations (away from the engine).