GIMP :: Artifacts After Flattening Two Multiply Layers

I've making a light glow texture for a game with gimp. I've created the light with supernova effect, then i used a a black to white gradient layer with multiply, also another multiply layer is used to make the light fainter.

(there are more in the picture, these are mipmaps for game that are smaller and more shinier to get realistic glow effect).

In game it will be used for texture of an additive material, and a little artifact is noticeable if i watch carefully. These artifacts are noticeable too in flattened image if i add more lightness to it. This is what i'm talking about that halo around the light. But on mask there isn't anything like that.