Be sure denoising is enabled for every LightSelect. (In IR of course, this will have no effect, as denoising can only run in a final render - but same applies for a beauty pass of course).
Be sure every light is assigned to a LightSelect layer, as the "Rest" layer for lights not assigned to a LightSelect cannot be denoised (the only part of LightMix that can't be denoised). Generally if your Beauty looks good, and then when swapping to the LightMix there is noise, most likely denoising was applied to the Beauty, but not to all the Lightselects.
And as Marcin notes, extreme adjustments can introduce noise, especially making any light source brighter - if this is an issue, best solution is to bake the LightMix changes back into the scene.
