With xNormal/MikkTSpace and Marmoset (and UE4) you'll need to configure the MikkTSpace plugin in xNormal to recompute the bitangent in the pixel shader. Also, in Marmoset, you'll need to ensure you've set the mesh tangent basis to MikkTSpace. Aside from that, it could just be a bit depth issue. Ensure you're saving out to…
The MikkTSpace Plugin in xNormal has the 'Compute Binormal in the Pixel Shader' checked when I double-click it to open the options - bit of a pain having to check this every time you bake, but I tried it before trying the Default settings. The Tangent basis in Marmoset is set to xNormal, too - so perhaps it's an issue of…