Mmm, I kinda think it's a bug because I left my lion thing to be cooked overnight on local machine.
Workflow was like that:
1) model, textures, etc.
2) small sized test render, post tab adjust, Lightmix tab adjust, render saved manually (i.e. I pressed save icon in corona buffer), much love to corona :)
3) the only change at render set up:common was render size to be bigger and render output to be saved by computer (beauty and elements)
4) morning, after way to much coffee - at corona buffer I can see my big overnight render with Lightmix values from small test run applied, but file saved on hard drive was not the one I can see in buffer - so I saved it again from buffer by manually clicking save icon and got right image, exactly as in buffer.
Small animation test over lunch rendered same result: buffer shows Lightmix applied and saving from buffer by hand - gets you right image (as far as i can tell), saving by computer - wrong image. I mean beauty pass.
Or am I just seeing things after long day?
I got bunch of elements saved both ways - will have to compare them later.