We are also facing this flicker problem from previously last few projects and its constantly appear in every second night shot.
we have tried all the way to resolve this problem (Path tracing, Simple UHD, 4K Cache) but still we have flicker in our render, for now we have only one way to get flicker free render is to put omni or standard light instead of corona light and use 4K Cache for UHD but this method cost lots of render time.