Ok, how the perfect reaction would be if you were at charge?

I understand people have sympathy inclination to victims, so everyone would assume the victim is good and other side is bad. I have worked long enough with japanese people knowing they can throw unpredictable tantrums.

As a manager, what would be your best course of action to deal with similar situation?

Acknowledging the mistake immediately seems like a good start, as I've said.

Life doesn't always have to be from the perspective from “a manager”, these are community volunteers doing untold hours of unpaid work. Just be a person, whose acquaintance is upset you replaced their handmade postcard with an AI-generated one.