It appears as though when adding material layers to a material instance that uses the FromExpression shading model, the subsurface color ends up white. If you add a DebugFloat3Values and output it to emissive in the base material, the material itself thinks it has the correct Subsurface Color value, but investigating the buffer visualization or pixel inspect shows the resulting value is always pure white.
Interestingly, this only appears to be an issue if the layer containing the subsurface shading model is adding to the layer stack in the instance window. If the layer is added to the layer stack directly on the Material Attribute Layers node in the material, this issue doesn't occur.
I was able to reproduce this issue at least as far back as 4.27.2.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-165303 in the post.