the smartphone is new, nothing happened, otherwise everything is fine, except for the banding
So it's happening to thumbnails of images/photos with dark color? If you post the direct link to the thread, some media might pick it up, which makes sony to fix the issue if it's a sofwware issure.
The lines go from the black edges of the images, in the photo after the black lines from the image there are lines further on the entire display, most noticeable on a dark background
If it's just happening to some images, it's most certainly corrupted image data, not the display itself. What if you open the image in other devices?