This is because the normals are computed from the new geometry after displacement, that is derived from the old real geometry, so old shading normals are disabled. This causes basically disabling of shading normals when using very little displace height/mostly black texture. A workaround is to subdivide the geometry before displacement. I tried to go further and do a spherical interpolation of the shading normals, but it gave unstable results. I'll revisit it at some point, but for now I've already spent too much time with displacement.
PS: it looks like vray interpolates the normals correctly, but not the actual geometry.