Banding

A gradient needs enough distinct values to look continuous. When there are too few, the eye sees stripes. The causes stack up: recording in 8-bit, heavy grading that stretches a narrow range, and compression on export or on the platform, all of which throw values away. Shoot 10-bit if the camera offers it. Go easy on contrast across a flat backdrop, and add a small amount of noise or grain before export, which breaks up the steps enough that they stop reading as bands. A plain coloured wall behind a subject is the shot where this shows up first.