Interesting. My Luminous flats were successfully taken at 0.011 sec with basically indoor ambient light. When below that time the image generated included horizontal lines.
Skywatcher 190MN - EQ6 Pro (with Belt Mod) - ASI1600MM-Cooled - ASI EFW7 - ASI120MM - WO f4 Guide Scope - Rigel nStep - KStars/Ekos - KDE - PixInsight