In https://code.qt.io/cgit/qt/qt3d.git/commit/?id=fb0c9421b1e172833f3ac2166b9a39bd83a8b6ef the phong shader was refactored to move the phong shading function into a common function reused across different shaders.

The incorrect definition of the shader was included however. Originally, the definition in phong.frag / phongalpha.frag / etc was

fragColor = vec4( ka + kd * diffuseColor + ks * specularColor, alpha );

note ambient "ka" color contribution is independant of diffuseColor

After the refactoring, the color is calculated using:

color = (ambient.rgb + diffuseColor) * diffuse.rgb + specularColor * specular.rgb

now ambient color contribution is scaled by diffuse. This is incorrect, and the original definition was the correct one to use.

I believe this may have been introduced because SOME of the original shaders used

fragColor = vec4( diffuseTextureColor * ( ka + diffuseColor ) + ks * specularColor, 1.0 );

eg diffusemap.frag. I'm not sure if that was originally intentional, or an existing bug which predated fb0c9421b1e172833f3ac2166b9a39bd83a8b6ef...