[rcheshire (Rowland)]

Just a few thoughts - Given that the bands extend across the dithered stack, processing seems a likely cause. Difficult to say, if you changed processing options as well as removing frames in that last image.

It is also possible that the frames you removed did not respond well to your flats. Normalisation, if used, may exacerbate the problem.

As a base line, you could try processing a small image set (to speed things up) with a sample of the frames you removed, without normalisation (if you used it) and with bilinear deBayer (simple and usually safe) to see whether the issue reoccurs - as basic as possible.