saw this happening with other images previously, but in this case I can tell for sure:
#700489
The picture I iploaded to e621 was directly generated by Photosohop from .psd
One on FA was same image which was re-coded later by website to a different color space.
So it' either same, or original version was "higher quality" as lossy recode always creates artifact and color space change in general changes look (noticeably on a phone browser because they have to limit 16-bit colors). Afaiik e621 also did some re-codng spreee recently. So is that the reason? e621 version was reduced in quality so it had to be replaced?