It seems that for me it is producing exactly the same result regardless if I use a single block, or a stacked number of layers.
I have two questions:
-Do you have UHD Cache enabled, or rendering using path tracing only?
-How is your SSS material made? Can you post a screenshot, or the scene itself? Does it have some kind of surface enabled? (e.g. translucency or refraction used?) If it has a surface, then you are comparing two different things: in case of a single block - light entering through a surface, scattering in the volume, and then exiting through a surface; in case of multiple blocks - light entering through a surface, scattering in the volume, entering through another surface, scattering in another volume, surface again, and volume again, a few times, depending on the number of blocks you have in your scene.
I also did not manage to get any "artifacts" or anything like that when using IR and regular render, and when applying one material, or copies of it.
Update: adding my scene - can you reproduce the issues in it?