That second normal maps image has that green line at the top of it, is that effecting it as all? not sure how your unwrap is stretched across the texture.
i took your normal map into Photoshop and color sampled the neutral blue outside of the baked part is 128,128,255. The part where you're having issues the color is 127,127,255
are you triangulating the mesh before export and using the same triangulation in marmoset?
also, after you offset the uvs, do you weld the verts back on?
Are you exporting tangents from 3dsmax? If so, 3dsmax creates some weird tangents on mirrored geometry and you will experience this issue in any game/rendering tool that is loading them. I would guess that if you un-check tangents from your FBX file this will go away.
Just did a test myself. No matter what I did I couldn't get rid of the seams with mirroring the model. I did manage to almost eliminate it by setting the mirrored parts to separate smoothing groups. That said you could always keep the large flat center piece unique so as to avoid the seams. Might be a bug in Marmoset 2.0, but can't say for sure.
Well if you're not getting the issue with other materials, or with no materials that would seem to suggest that its the normal map itself that is somehow at fault.
Try baking a map inside of max and setting the tangent space to max in toolbag to see if that helps.
Try placing the seam in the corner instead of the middle of the large faces. I don't ever really have this problem but I always avoid seams/mirroring down the middle of big flat faces cause the repeating is usually noticeable ingame.
Maybe? Again I usually avoid splitting right down the middle of flat faces like this so I wouldn't really say its common.
Oh one other thing to try, you said you split edges/smoothing groups at uv borders, do you have it split at the mirror seam as well? Try keeping that as one smoothing group instead (or trying the opposite if it is already).
^ Pushing the seams to corners, that's a good idea. Always avoided it and put up with the mirroring butterflies because I feared it'd screw with the shading on corners, but I'll give it a try on the next asset.
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I have had some problems myself inside toolbag2, which can be related.
also, after you offset the uvs, do you weld the verts back on?
Try baking a map inside of max and setting the tangent space to max in toolbag to see if that helps.
At worst this should shift the error to a less noticeable spot.
Oh one other thing to try, you said you split edges/smoothing groups at uv borders, do you have it split at the mirror seam as well? Try keeping that as one smoothing group instead (or trying the opposite if it is already).