Try using these console commands to turnoff tonemapper:
r.TonemapperGamma 0
r.TonemapperFilm 0
r.Tonemapper.Quality 0
If you use them in game the Show->PostProcessing->Tonemapper from editor will stop working, but i guess it will resolve your in game issue (I just reset editor and it works again as long i don’t PIE and throw the console commands again)
Using these 3 rather than ShowFlag.Tonemapper 0 i got the same result within a Shipping Build