Multiresolution subdivide modifier on a mirrored surface causes a visible seam I've been following a tutorial by Daniel Kreuter on Youtube (https://www.youtube.com/watch?v=wdsY4BtlcpQ), modelling a body using a mirrored surface.  I thought I had followed his actions fairly accurately, but I'm getting a weird seam on the mirrored plane in my model when I apply a multiresolution subdivide.  Oddly enough, it doesn't appear when I use the subdivision surface modifier.



I'm not really sure where to start looking for problems.  This question has an "answer" which seams (see what I did there?) to have the same problem I'm having, with no solution.  This forum post looks like they have the same problem.  One of their suggestions is to move the mirror modifier above the subdivide, as the modifiers are applied sequentially.  Unfortunately, that gives me the error "Cannot move a modifier requiring original data".  The other suggested fixes, checking clipping (it was already checked) and increasing the merge limit,  haven't worked.

Another thing I've checked, based on what I've seen on the web, is whether faces have been inserted along the mirror plane by accident.  Sadly, that's not the solution either, nor is it due to duplicated vertices.  What should I try next?
First I presumed that you did mirroring by negative scale around YZ-plane, then you have flipped normals at second half. But then I noticed modifier, so I was wrong.

I noticed similar result if Subdivision modifier placed higher than Mirror. Also Blender prevents from moving Multiresolution with subdivisions under Mirror, saying "Cannot move beyond non-deforming modifier" or "Cannot move above modifier requiring original data" for moving Mirror up. 

I think it's a way Blender modifiers works with mesh data. I took a peek in official docs and found this: https://www.blender.org/manual/modifiers/the_stack.html - according to official info modifiers applied in order from highest to lowest. When I tried to apply them in this order, I also get seam in final mesh with both modifiers, but it was just crease in geometry, vertices are wielded, normals are outside, just little wrong geometry. Old edge at section at half-mesh left unsmoothed by modifier, because it processed differently areas where edges belong to just one face.