Generative AI expanded canvas - incorrect brightness for fill

There is distinctive vignetting on the left edge. The fill is just continuing that vignetting. The color profile is what it is, that doesn't come into it.

It likely for this reason. "generative fill" works on the cloud

it must do a color profile conversion, ("document profiles" are things like sRGB AdobeRGB etc, simply put they are descriptions of the colour characteristics of an image, and mean that the pixel values "mean" different things in different profiles. Areas can be converted or "assigned" to different profile )

It's likely converting your new "generative fill" section to a different profile for the 8 bit "inpainting" technology to be applied. And then it is converting back to the source profile of your document. When it comes back there is a mismatch.

A skilled color correction expert, like me, could easily match the numbers of the adjascent areas for you 

I love the picture it looks great!!!

I can definitely see what you mean, and all the more so after copying the lower screen shot into Photoshop.  It would help top know exactly what your selection covered.  I tried deleting everything left of the tree (see layer thumbnail) and selected the empty space plus about ten pixels of the image, and it worked more or less OK for me.  You can see a darker column in the centre of the canvas.   I'd say that the Ai worked out the lighting as the foreground would be unlit if this was real.

p.s. I tried a bunch of variations and all of them had this issue.