in my first reaction to @mauricetoet‘s question in this post I explained everything I think. As everyone could see, Maurice was able to perfectly calibrate his f/2.8 DSLR data with flat & light frames which had different iso values
Your explanation was perfectly clear to me, so I don’t know why others are struggling. Before actually stacking the data, you can get instant feedback from APP to check if the calibration files were probably prepared. I cannot imagine a clearer warning signal.
BTW. If you use darks, I won’t bother using a masterbias to correct the lights and darks (as the bias offset is both in the lights and darks). To keep things simple and clear, just bias-correct the flats.