Dear @keesscherer, you will need to create the masterflat out of bias calibrated flats.
Loading a masterbias and masterflat will not cause the master flat to be bias calibrated and it shouldn’t.
All individual flats need to be bias calibrated when you create the masterflat. So all flats need to have a b in the frame column before you create the masterflat.
I’ll be happy to demonstrate it if it’s needed?
Mabula