According to the FCOM, AFDS failures affecting only active modes section:-
• the autopilot remains engaged in an attitude stabilizing mode
• an amber line is drawn through the mode annunciation
• the EICAS caution message AUTOPILOT displays
So with dual FMC loss I thought autopilot reverts to ATT mode (albeit not annunciated on FMA). I know on initial engagement autopilot/flight director logic with no prior selected modes is into HDG hold with < 5 deg bank or ATT if >30 deg but always thought the above solely applied when both FMCs fail? Good question posed.