Thank you and I'm glad you noticed it because it's
good for you ! If the original Japanese version included "san", it made me strongly believe they're not dating. LOL
Because only the couple with different age or social status called their partner "san". People at the same age use "san" before becoming a couple and remove it after becoming a couple. Shinobu is 5 years older than Sakura (21 years old as Fumi) so he'll always been called "san" even after marriage. But I don't think the fucboy is as old as Shinobu. I think he's the same age as Sakura and the blond.
So, if Sakura and fucboy are close friends or dating, Sakura must not call him "san" as you can see in the other romcom manga. When you calling someone the same age or same social status as you using "san", it means you're not close to the person. Using "san"
remind yourself how you look at the person and there is still a gap in your friendships.
You can call your new friends "san" but when you're close to them, they'll ask you to stop using "san" with them anymore, right? LOL
If Sakura call the fucboy "san" in front of him, I'm glad for you
@diphuc1 , the author's intended to show us they're not dating. LOL
NOTE : Shinobu also called "Fumi-san" since they start working. So, "san" is also use with colleagues to show respect. If Shinobu become a couple with Fumi, he must call her Fumi-chan like he called Sakura-chan.