Between loss of generators and APU online you will only have battery power. This means loss of one (right) ADIRU and one DEU. I know out of experience that in both cases the FD will disappear as soon as you change from ALT HLD or VNAV to a descend mode (GS, V/S, LVL CHG or VNAV PTH/SPD). My guess would be that the FD needs two independent and valid signals from both the display computers and ADIRU's.