What this likely is, is softened normals for the edges. You can select the originally border edges and go Normals > Harden Edge. That should straighten it out.
(oops, didn't read that you already did that)

Maya has to guess whether an edge represents a corner, and the faces should have a hard edge between, or whether this is a smooth surface and it will try to adjust/interpolate the normals to simulate a smoother surface, which is what is happening here it looks like. It's just an artifact of Maya trying to smooth the normals out.