Thanks for your message. At present Corona Lightmix doesn't support lights distributed with RailClone. In order for this to happen, the Corona team will need to add support using our RC API as they did for Forest Pack. Until then, as you have spotted, we get around the limitation by cloning the lights before render time and deleting them once it has finished. Hopefully, full support can be added by the Corona team soon.
If LightMix is essential, a workaround might be to instantiate the lights into the scene using RailClone tools.