Looks to my eye like some 8-bit underflow artifacts that I've seen. And the way the line follows the gradient reminds me of when I've broken a gradient-domain

Looks to my eye like some 8-bit underflow artifacts that I've seen. And the way the line follows the gradient reminds me of when I've broken a gradient-domain Poisson solver to reconstruct an image from gradients. Maybe they're decorrelating the image data via gradients to compress better and then compressing those along with a sparse set of primal pixels? (See, e.g., http://graphics.cs.cmu.edu/courses/15-463/2019_fall/lectures/lecture9.pdf http://graphics.cs.cmu.edu/courses/15-463/2019_fall/lectures... for a nice overview of gradient-domain image processing.)