it seems as though furuta isn’t able or allowed to use the term “father” when addressing his father…so it’s possible he calls him by his surname or another title instead. it hasn’t been revealed if it’s due to hiding their relation, for the sake of formality, or other reasons.
Honestly at this point I wouldn’t be surprised if Furuta was also secretly a mafia boss, a stripper in a gay bar, Yoshimura’s coffee beans supplier, the guy who asked for a selfie at Eto’s book signing, the girl with freckles at the CCG information desk, the guy who’s looking at me weird as I type this post…