Drop your movie on the timeline, add Lumetri color. Drop Whites down to minus 20 or so and you will see your masks. Bring up Lumetri Scopes and you will see the difference in white levels on the scope display. Now delete Lumetri Color. Try this: Drop “VIDEO EFFECTS/LEVELS” onto your clip. Bring RGB White Output down to 200. Output your movie. Bring that movie back into the timeline and apply Lumetri and bring down Whites again. See if the hard mask lines are gone.