[avandonk]

I have found that taking darks just after your flats really helps. You should correct your flats with the darks in the usual way.

Think about it the dark for your light removes the bias as well. The flat still has the underlying bias DSLR banding pattern. You are then modulating all your light data with this artefact.

Try it!

It is really most noticeable with really faint data and extreme stretching.

Bert

Below is a crop from the corner of a one second exposure (dark) at 100 ISO and 22C stretched to show detail.