After some researching, I have a look into the Unreal Source files, and found this line in PostProcessing.cpp:
const EPixelFormat SceneColorFormat = bProcessSceneColorAlpha ? PF_FloatRGBA : PF_FloatR11G11B10;
If r.PostProcessing.PropagateAlpha is set to 0, there is something, that leads to banding issues. So I set it to 1 in DefaultEngine.ini, and there is no more banding, alternative way to set it is through project setting, it called “Enable alpha channel support in post processing (experimental)”. But I still want to know, what exactly causes this banding, so I can solve this problem without disabling separate translucency.