Thank you @Ozykz for the info and the very useful suggestions. Mine was 5.3. For sure it will work better in UE5.4, I hope.
PS: Vray for Unreal allows per object? Why are you not using it? (Never used it before, so I want to know advantages and/or disadvantages, basically)
Nice to see you here @Luoshuang ! 
Actually those white walls (maybe it could be a good idea to modify the default material, with a more real-world white value) had the default value (0.9?) but I have tried right now with a value of even 0.7 (and I was ready to be wrong before, getting white lighting now), but the result it’s almost the very same; super green bounces and almost no shadows. Also tried with path tracing and it’s wuite green too, but not so exagerated:
Here you are the scene to test: 221.38 MB file on MEGA
Thank you (both) and best regards!