Normal map from SB causing strange shadows where they shouldn't be

I'm creating extremely low poly mirrored cliffs in ZBrush and texturing in Substance Painter.  This is the set of mirrored cliffs in-game with no normal map:

You can see the seam where the two sides are mirrored.  I have some artifacts but my main concern is how this model looks when I add the normal map:

What is causing that grey "shadow" all along the edge of the model?  This is what the normal map looks like (front is the top island):

I can't figure out why this is happening.  I'm using a 2-sided panoramic environment map but I had the same problem without it.  I'm really stumped, anyone have any ideas?