Off the top of my head, I can't think of any Corona setting that would do that :) Maybe someone else can think of something though. For me, it was most likely Max - depending where the scene came from (if it was from an older version of Max), Max used to have separate Input and Output Gammas, but new versions do not give you access to each one independently any more so any set incorrectly can remain "locked and unchangeable" in its incorrect form (you can see the values, but they are greyed out and can't be changed) and I wonder if that is what happened here. Glad you got it resolved whatever the cause!