Responses
I think it's this:
https://www.mangago.me/read-manga/souteigai_no_sweet_marriage/
i also found this other one with a kind of similar premise in the way that theyre fake dating but one of them is secretly in love with each other but a bit different in some aspects
https://www.mangago.me/read-manga/koibito_keiyaku_chuu_ni_tsuki/
it's about two office workers who end up getting into a fake engagement to stave off romantic attention they were getting. one of them was actually in love with the other and proposed the engagement to get closer to him i think? it's been a long time since i've read it, so the details might not be accurate.