Not fully sure I understand the question, sorry. But some thoughts (if I do understand right :) )
- Be sure denoising is enabled for all LightSelects
- When turning off a very bright light source, such as the sun, there may be more noise in the other LightSelects where this light source was present - this is due to the Adaptive light solver, which can only assess the overall scene. Since there was a bright light source at that part of the image, more processing power was given to that light source, since it was the most major contribution to that part of the image. Less processing power was given to other light sources in that area. So, in LightSelects, there may be more noise in that area.
You can check out the "I can see persistent noise!" paragraph on
https://coronarenderer.freshdesk.com/support/solutions/articles/12000057961-how-to-set-up-multiple-suns-environments e.g. one possible solution there is to disable the adaptive light solver.
Let me know if I am off-track though and not understanding your question :)