Honestly I don't see anything wrong and I even zoomed way in. I think it's just how the light is catching those pebbles of the caviar. And besides, it's under the flap anyway. I wouldn't worry.
I have a much worse issue (pinched stitching on the front above the little flap that holds the hardware on one side. It's like they maintained a perfect distance from the edge all the way and then just lost concentration for a second and went closer to the edge. LOL I kept the bag. I asked others if they noticed it and nobody did until I pointed it out and even then they all said it's barely noticeable to them even after I showed them. Since medium black caviar cf are hard to come by I kept ot since I don't know when another would have been available. Mine is a very recent purchase with the metal chip/plaque. Point is we may be going into wayyyy to fine of detail and finding flaws that nobody else will ever notice. While we may be perfectionists, these are really still just handbags and there are bound to be small deviations on every single bag if you look hard enough. Enjoy your bag!