Why "Father Roberto" but "Father Hiraga"? In Vatican Miracle Examiner, the main characters are often addressed as Fathers Roberto and Hiraga. Thus, Roberto is addressed by his personal name, whereas Hiraga is addressed by his surname. (Hiraga's full name, using English-style ordering, is Josef Kou Hiraga. Josef seems like a name taken upon entering the priesthood.)

Why is there such a discrepancy? What I've seen in the past (e.g. in The Brothers Karamazov) suggests that at the very least, it's not unusual for priests to be addressed as Father + $personal_name, and Hiraga's form of address does not fit that.
It seems this is a highly speculative question.
From research, I found out that the formal way to speak to a priest is  Father + Name + Surname.
Still, as is, those are not mandatory rules, and the priest might prefer to be called a specific way and say so to his congregation.

So, the discrepancy between Father Roberto and Father Hiraga could be because of his personal choice.
Or because people read his name in the japanese order and see Hiraga as the first name.
It also might the decision of the writer to apply japanese adressing etiquette to the way people call the character.

I don't think a definitive in scenario answer will be possible.