1. Make sure denoising is enabled for each LightSelect layer
2. Make sure you are not making changes that are too large in LightMix, e.g. reducing or increasing light intensity by a factor of 10 or more as a very general guide (you can often get off with using larger changes, but that does depend on the scene, so if you are making adjustments larger than that do some tests to see if that is what is introducing the noise)
Hope this helps!