That looks like more than a 100% crop -- maybe 200%? So if it's not going to show when the image is used can we be happy and live with it. That's the founding principle of JPEG after all and we're all pretty happy with JPEG. If you can't see it in use....
This is a screen clip from 16 bit TIFF file that was being edited in PS that was enlarged to 200%. Yes the clip itself was saved as a JPEG, but it's a close representation of what I was seeing on the TIFF, which was created in LR from the DNG file.
It also looks like what happens when not enough tonal data is spread over too wide a tonal space.
I think this may be the answer. This image had a tight crop to begin with, followed by some heavy handed processing in PS Levels. Maybe a cumulative effect taking place?