Without being able to review the specific TC data, my presumption is that the abnormal FMEA considered only full thrust takeoff for the particular aircraft you are considering. The inference is that there is some consideration with flex which creates a problem with the numbers .. for the life of me I can't imagine why ?
Derate IS the certification full thrust basis so that thrust would have been considered in the derate FMEA.
One needs to keep in mind that flex and derate are totally different animals from the certification viewpoint.
There are a couple of other considerations in the back of my mind .. where does the restriction to which you refer actually appear ?