Thin Black Outline!

Why Ai adds a thin black outline to the shape (with a solid fill colour) after adding a gradient fill (Appearance Panel) to it!

I have a shape with a fill (solid) colour (img 1), adding a gradient fill (blending mode: softlight) to it (img 2), then it adds a thin black outline to the shape!

I add a gradient (black and white, softlight) to change only the fill colour not the gradient attributes (if I decide to change the shape's colour, the shade is fixed, the fill colour changes).