Lightroom classic colour banding with JPEG exports

The jpg format uses lossy, destructive compression in order to create small files.

A low quality setting will result in a small file size with visible compression artifacts. Flat areas, like blue skies are prone to banding.

A high quality setting will result in a larger file size with few compression artifacts.

A quality setting of 85 is considered optimal for image quality. Using a higher setting will not improve image quality, but the file size will increase.

Note that even at quality 85 or higher, there is still some loss of image quality, but it may be hard to see.

I have some images with cloudy skies with a lot of fine gradients, where I get color artifacts even at 100 quality.

For more information on file formats, see https://perberntsen.com/misc/technical/fileformats.php

Here's an article on jpg quality settings in LrC: https://regex.info/blog/lightroom-goodies/jpeg-quality

Quality 0, 78 kb

Quality 30, 142 kb

Quality 60, 229 kb

Quality 85, 796 kb

Quality 100, 1.16 mb

Specify these settings. "I am using the same settings I have always used and never had this problem" means nothing. As you now use a pro Mac laptop, you now have a screen that is way better than the screen of your previous laptop. That means that what would get unnoticed on that old laptop, is now clearly visible.