Can flattening a PDF cause a color shift?

Hello all,

I've got a client who is self-publishing a series of books via Amazon KDP. The cover design uses the same colors across all the titles with just a change of image on each book.

For all the volumes, I've exported a PDF of the cover art and all was well. For the latest volume, KDP told my client the PDF should be flattened. I did that. But when she got her proof copy back, there was a notable color shift in the yellow we're using.

I checked my files and there was no color change. The color values show the same in Acrobat in both the flattened and unflattened versions of the document. They look the same on my screen. But they clearly don't look the same when KDP prints them.

Is this a known issue? Or is it just a coincidence that it's happening now and is actually caused by something else (e.g., some other change on the printer's end)? My client asked KDP about it and didn't really get any useful info.

Any thoughts?