We'd need more information. First, please don't say "latest" because what you think is the latest may not be - please always just say the specific version :) So, do you mean Corona 8, 8 HF1, or 8 HF2?
Then what do you mean by denoising not working in lightmix, is it not working for any of the light select layers, is it not working for just one specific layer? What layers do you have set up? You can check each light select individually to see if denoising is having an effect. Seeing some render, and some of the layers, and some screen grab of your light select set up would be useful too. What render target did you use, time, noise, or passes, and what was set for that? What denoiser mode did you use, Corona, NVIDIA, Intel, or Gather for Later? It might also be worth sharing the scene (support.chaos.com and "Submit a request" to share it privately with us, rather than publicly on the forum).
Thanks for the additional info!