hmm you make a good point what im trying to say is if he looks trough the shield the flames will automaticly get summoned on it cause its in the way even though its a spiritual weopen it can still be seen and we cant say for sure if theyre appearing or traveling with ridicilus spead but Ill agree theyre getting summoned cause kishi is hinting it
Eh, I think that it spawns on whatever it is FOCUSED on. If the user is focusing on the ninja behind the shield, then the ninja behind the shield will get hit. If the user is focusing on the shield, then he will hit the shield.
By your logic, Sasuke cannot use Amaterasu while inside Susanoo, because then he'll just be hitting the inside of Susanoo.
susanoo is spiritual energy but yata mirror is a spiritual weopen it changes its atributes acording to the attacker he can simply look trough his susanoo and hell he can simply look trough the shield BUT the shield will change its atributes which will stop the amaterasu to be DIRECTLY summoned on sasuke and get summoned on it instead. The susanoo doesnt have this ability hence sasuke can just look trough it. Like I said before maybe he can look trough it but no way the flames themselves are getting passed that