When I use generative fill in an area that overlaps a plain color field (e.g., a background/backdrop), the fill is always several points too dark. I have to go into levels and raise the midtones 2-3 clicks.
it is hard to tell what exactly in your snippet is the Fill and what the part of the picture. I assume you mean the white part besides the hair? For me it looks good. You can always try to lighten the whole thing in the traditional way.
Pay attention to what adjustment layers are active, and the exact placement of your genFill in the layer stack. You're describing what happens when you add genFill in the wrong place in the layer stack, so I suspect that's the problem. As a general rule, apply a genFill while targeting the top layer. This ensures that the AI riffs off the current state of the composite and not a subset.
There were no misplaced layers. My gen fill layer was on top of all my layers, and then I put a down-curve on it to demonstrate the issue. Please give it a try yourself.