Diffuse color has already been calculated relative to the specular, so use...

Diffuse color has already been calculated relative to the specular, so use that directly to fix dark fresnel for indirect specular lighting.

src/renderers/shaders/ShaderChunk/lights_physical_pars_fragment.glsl.js

@@ -128,7 +128,11 @@ void RE_IndirectSpecular_Physical( const in vec3 radiance, const in vec3 irradia
 
 		BRDF_Specular_Multiscattering_Environment( geometry, material.specularColor, material.specularRoughness, singleScattering, multiScattering );
 
-		vec3 diffuse = material.diffuseColor * ( 1.0 - ( singleScattering + multiScattering ) );
+		// The multiscattering paper uses the below formula for calculating diffuse 
+		// for dielectrics, but this is already handled when initially computing the 
+		// specular and diffuse color, so we can just use the diffuseColor directly.
+		//vec3 diffuse = material.diffuseColor * ( 1.0 - ( singleScattering + multiScattering ) );
+		vec3 diffuse = material.diffuseColor;
 
 		reflectedLight.indirectSpecular += clearCoatInv * radiance * singleScattering;
 		reflectedLight.indirectDiffuse += multiScattering * cosineWeightedIrradiance;