When you subdivide with SMT disabled for the first couple of subdivision levels in order to preserve hard edges, that can throw things off when you eventually bring the mesh into an external 3D app for rendering with displacement. Is there a work around for that?
I think it has to do with a discrepancy in how ZBrush is subdividing the geometry vs how the 3D app (in my case Max) is doing it. I suspect it may even be pushing the displacement map too hard.