Be sure that each LightSelect has Denoising enabled. This is enabled by default, but if it is off for any of the layers, then the final composite LightMix pass will not look correct. Some screen or video grabs would help, both of the final result, and of your set up in Render Settings and Render Elements. Also please say which version of Corona (full version please, not just "latest"), which version of Max, which OS and version. As a note some earlier versions of Corona did not allow the "Rest (unassigned)" layer to be denoised, so if you have anything left in that, then that could be the problem (try to put every light in an actual LightSelect in that case, so that denoising can be enabled for it).