I don't see this as a problem actually. My hair line at the nape simply grows in thick at the sides of my neck. If I wore a short cut, that's what I would edge up. But since I wear it long, I get a natural hair line, and a hem that represents the line.
If the hair at the sides of my nape is fills in on the skin lower down my neck, the hairline will reflect that shape. Also, the hair in the middle of that place comes from my crown, way up on the top of my head. It makes sense that there would be a gap. If I get a blunt cut, an edge up or an under cut, a year later it would look the same.
