I have been led to believe that to everyone who doesnt know him as uchiha madara his sharingan casts an auto genjutsu to give them familiarity. I think thats why amaterasu activated when sasuke saw his sharingan. Maybe to prevent him looking like a family member or someone he might automatically trust. I think thats why noone was ever like oh the uchiha race cant be extinct if there's an uchiha as mizukage or what have u.