SKEffectNode shader bug

Question

I'm developing a game using Sprite Kit and Liquid Fun (Google's implementation of Box2d with liquids). After I applied a threshold shader to make the liquid more real, I stumbled in a strange behavior that can be seen on this video: Current Behavior

The way it should behave can be seen on this video: Normal Behavior

I'm applying the shader on a SKEffectNode that contains all the particles (SKSpriteNodes)

That's the code on my threshold shader:

void main()
{
 vec4 color = texture2D(u_texture,v_tex_coord);

 if(color.w > 0.4) {
  color = vec4(18.0/255.0, 122.0/255.0, 232.0/255.0, 1.0);
 } else {
  color = vec4(0.0, 0.0, 0.0, 0.0);
 }

 gl_FragColor = color;
}

Does anyone have any clue of what is going on?