HDRI Sky Cubemap won't update

I want to procedurally generate a cubemap and then assign it to a HDRI Sky when my scene loads:

 void Start()
     {
             Cubemap cubemap = GenerateCubemap();
             HDRISky sky;
             if (vol.profile.TryGet<HDRISky>(out sky))
             {
                 sky.hdriSky = new CubemapParameter(cubemap, true);
                // A simple sphere to test if my cubemap is generated correctly, does indeed work    
                testSphere.material.SetTexture("_Tex", cubemap);
             }
     }

However, the sky does not update in the scene, independent of the Update Mode setting of the HDRI Sky. It does however update to the correct sky when i change something in the script and Unity recompiles during play mode.