I know we didnt get to see any action in this chapter but... darn I LOVED it!!! specially since Kubo didnt take the expected route, and instead surprised us all,
I guess if Isshin wouldnt be able to see hollows / shinigami , then that explains why he couldnt save Masaki from dying, (not being able to read any hollow/shinigami signature / prescence) and even if he was there, he would have human strength and wouldnt have been able to fight even a hollow (I guess) plus supposedly it wasnt Grandfisher, but something/someone bigger.
Also if what my2kb22 says is true (it seems possible) , then that would explain why Isshin has powers again, because Masaki died and that would finally allow Isshin to be Shinigami again. Wow talk about loosing ties, we definitely got a lot of answers with this chapter alone.