Interesting, but we would need the scene and solid reproduction steps to understand it. Can you try putting it on your Dropbox, Google Drive or any other hosting, and sending us the link?
Also, is the noise visible ONLY in the LightMix after cranking up the intensity, or also in the original beauty pass? Persistent noise is expected when increasing intensity in LightMix too much, but it should not be dependent on the light intensity units anyway.