Corona is adaptive in terms of how it samples lights, giving priority to lights that have the most impact on a particular area. This means the sun takes priority, being the brightest, so other lights in that area where the sunlight land get less processing/attention. This means if you remove the sunlight, you will find the area noisy underneath because the rest of the lights weren't given as high a priority.
If you intend to use LightMix to truly control the end result, it's often best to make all lights a similar or identical intensity (including the sun), and set them to white, to give maximum control in the LightMix result.