In some cases, the banding effect may appear when applying the noise filter to images with faint brightness gradients. This effect is quite rare for normal images, especially when viewed on a true color display. It can be more visible on hi-color displays. This is a common problem of hi-color displays. If the display doesn't have enough colors then the image can have some bands of the same colors. Dithering is usually used to mask this problem on such displays. An original image usually contains some noise, which acts like dithering. When Neat Image removes this noise, the underlying problem of banding may come up again. A solution is to use a true color display or a better image viewer (in hi-color), which applies some dithering automatically.
To avoid banding try to reduce the noise reduction amount for the high frequency image component to 50%.