Testing CR9 RC 2 with max 2021 currently, and it seems that using 'Show with original postprocessing' in the VFB history switches tone mapping off completely.. or even darkens output, not sure, but it's definitely not using the tone mapping nodes used when image was saved to history.
This is an older v7 scene and tone mapping was not reset, maybe that's the problem. But in any case, it should work with whatever the tone mapping config was when the image was saved to history.