I actually tried to read the novel since I can't wait long to what will happen next. And I think you missed some parts because it's being implied a lot not just too explicit in the novel.
Maye they just gloss over it and treat him as having larger chest than normal. There are many times that the ML ask him if his chest getting big or when they're having a custom suit and the ML ask him if he needs a chest bind and giving exact cirumference of his chest. So yep, it's not the artist, it's the author.
I just re-read this part after not being able to wait not too long ago... Σ(  ̄□ ̄||) I don't even remember that part being asked. I know he said he'd make a suit. I'm not fluent however, so Google translate could have changed some words that lead my brain to autocorrect the sometimes wonky translation.
Wondering if the artist misunderstood or is just adding again on their own ( ̄∇ ̄") Cause he had no boobs for this arc in novel. I remember him checking and being like he's going for my chest again like he did before. Or maybe they made a mistake cause he's been flat for all this arc and the servant should have questioned it I feel. ( ̄へ ̄)At first, I was wondering if he was intentionally like pushing his chest together that it looked like cleavage, but it looks too big to just pass off as normal flat chest without being muscular.