This is the known limitation of LightMix, that if you make significant changes to lighting then you may find noise (which is why we don't recommend "day to night" style changes or similarly dramatic shifts in lighting). So, turning off or significantly turning down a very bright light source will leave noise where that light source used to be. This is because Corona intelligently samples that bright light source more in those areas since it contributes more lighting to those areas, and samples the other light sources landing there less - so when you turn off the very bright light source, the weaker light sources have had less attention paid to them and are noisier.
Solution to that is to make all the light sources pretty much equal in intensity - this will result in a very strange beauty pass of course, with the sunlight being no brighter than other sources of light like the env light or light bulbs (or, alternatively, the env light and lightbulbs being just as strong as the sunlight), but that's ok as you are setting up the scene for LightMix. That way, all light sources get a fairly equal amount of processing power applied to them.
