Shingo became insecure with their relationship and Keichi's feelings.
The scent disappeared not because kei was unfaithful. The scent was there but shingo could not smell it because of his insecurities. He thought kei was unfaithful and was scared of losing him so he locked his heart away in a sense and could not smell the scent. He smelt it back when he got a commitment of the ring and reassurance from kei and trusted him again.
If that's the case, then the mangaka is contradicting what she wrote in vol. 1, as well as what the brother said. In fact quite a few events of vol. 1 won't make sense. Not that it isn't possible, authors often lose track of what they have previously written, especially in a long running series like this.
Sorry- don't know how to edit my earlier reply (never needed to before)- meant the very first ch.s of the whole story, not vol. 1 of this book.
:)
so why did Keichi's scent disappear? i'm still not clear on that