The soft edge doesn't use the gradient for its... uh... edge.
The soft edge uses the base color of the shape for the... soft edge.
The gradient doesn't use the soft edge either... so... it has the hard edge at the bottom where the black is against the white of the soft edge.
Any place the base "white" fill of the shape transitions to black will give a hard edge. If you look closeley at your image there is a hint of an edge at the top.
I agree this is kind of a bug.
To get this effect without an edge just use the gradient by itself and blur the layer.
This works in this one type of situation but alas... doesn't address the overall problem.
-vern