22(Demon Slayer)

An Au where X and Y character have been dating/married for a long time but almost everyone is oblivious to it even if the couple does not hide that they are in a relationship.

As an example it could be Gyutaro and Zenitsu(in coffee shop/tattoo shop au or something like that) who while not making our in front of everyone or anything too extreme, do not hide how soft/shy they are when together and even hold hands or hug in front of others but for some reason no one makes the connection so at some point maybe Gyokko or Douma believe Gyutaro should finally get in a relationship and force him into blind dates(which Gyutaro believes it is just his friends trying to make him be more social) while Inosuke and Kaigaku tease Zenitsu over being single but it flies over his head(mostly because Zenitsu is happily married and normally Murata is also in the same room and while Murata has a crush on the teacher, Giyuu, who  visits their ship frequently he never made a move on him so he assumes they are teasing Murata).

And things continue like that until Ume decides to make a big party to celebrate that TaroZen are making N years of marriage/dating and invites all of their friends and even other people(so the writer could add Giyuu asking Murata on a date with Murata saying yes and passing out the next second or just them confessing) where everyone is shocked to find out the truth and demand to know why they were not informed only to be told that the couple never hide their relationship and assumed everyone knew about it.