Found the reason why. When opening, corona defaults to the beauty pass. It goes to the lightmix pass once you tick off and on a lightmix layer. So when you do a render after closing corona, what you see is different.
If you open the scene, start rendering, and then go to Lightmix, without editing any lightmix related thing, it should show correctly what lightmix element is supposed to show. There should not be a need to turn on/off anything to properly display the lightmix element. If you open the scene, start rendering, go to lightmix and it is not showing the proper lightmix, then it is a bug.
VFB defaults to beauty at the first render after opening the scene - I think this is better than displaying lightmix, as beauty is the actual render, lightmix is an element, like alpha or zdepth.