It's hard to see in the first set of images, but the second set is much clearer. In the WebP example, look to the right of the subject, about 1/6th of the image

It's hard to see in the first set of images, but the second set is much clearer. In the WebP example, look to the right of the subject, about 1/6th of the image's width from the right edge. There's a hard transition between shades of grey. The JPEG version directly above it also has banding but each band is narrower so the difference at the edges is more subtle.