Without more information, it’s difficult to say exactly why you are experiencing this issue. However, some possible explanations could include conflicts between DFAO and other post-processing effects, or limitations in the hardware or software you are using.
It’s could be as you have observed, the options available for DFAO may be limited depending on the Scalability GI settings you are using. This is likely because the higher GI settings in Unreal Engine with Lumen are designed to provide more accurate and realistic lighting, and may not be compatible with certain types of post-processing effects like DFAO.
What kind of different results do you get when you change the GI types?