RGB values of pure white over solid pure white should render to pure white regardless of the alpha values of the upper layer.
However in M8 a pixel which is RGBA 255,255,255,127 will render grey when composited over white.
Attached is an example showing a PNG with white shapes with antialiased edges over a solid white layer. M8 renders it with a dark outline. Photoshop renders the same thing as pure white.