Can't get generative fill to work on stitched images

Hello,

I scanned a photo in a magazine, that spanned over two pages. I stitch them together as best as can, leaving but a tiny amount of the white border. Before the generative fill, I would have to spot heal or content-aware fill by sections of the seam at a time. Now with the generative fill, I'm hoping to simply have to marquee select, hit the generative fill button, and let it do the magic.

The problem is, the fill keeps on generating the white border. The spot healing tool works fine, but the fill is so much better, not to mention more time-efficient.

That being said, I have a feeling it doesn't work because of the way I stitched those two photos together. Layers are merged and flattened, so I'm not sure what could be the issue...