I don't know what to tell you. I re-exported the pictures and now the Jpeg are good. Is it because I have built the 1:1 previews for all of their NEF? Or because I applied a standard screen Output Sharpening during the export? Or something else? I have no idea.
The difference between Library and Develop Preview at 1:4 still persist in several pictures that had noise adjustment or unedited, shot at ISO 640 or above, but not all of them.