SSS, multires, and normal flipping glitch(?)

I’ve made a sash that wraps a figure and is to be rendered as if made of alabaster, just a long narrow plane made to spiral around the figure and draped over it using the cloth modifier, which was then applied to the sash mesh. So far all aces.

I’m using 3 levels of multires to sculpt the sash, enhancing the folds and countours suggested by the cloth sim shaping. The alabaster material uses a very high-scale SSS setting to get the translucency of that stone. But when rendered, part of the mesh has its normals facing the wrong way for the SSS to work right:


The section of the sash in the FG displays the “inside” of the mesh in terms of the normals, and the SSS doesn’t work properly for that reason, although it does seem to be OK in another area (small inset and upper right of the full-object render). Image at bottom right has no SSS.

Easy fix, right, just flip the normals for that portion of the mesh. But that doesn’t work while multires (MR) is applied. I found that flipping the normals at one MR level does not flip them at all MR levels, and furthermore that switching from one level to the next makes all normals revert to their original orientation when MR was first applied. The normals appear flipped in the UI (Edit Mode), but render as if not flipped, even when the MR display and render levels match.

The only way I could fix this is to apply the MR, and then flip the normals – that works as it should. But it also means that versatility in making edits to the sash sculpt – the reason for using MR – is lost.

Does not being able to flip the normals while using MR sound like a bug, or is there some specific reason why this does not work as might be expected?