Let me guess they are gonna "sleep" together. This will not cause them to swap but as soon as they stop "sleeping" together they will start swapping again. This will cause huge trouble and mmc finally realizes he's no over her yet.
Damn, total nothing burger ending. Guess it's better than random time skip endings.
Edit: author is the same one who is doing "starting today, we're childhood friends" maybe he got burned out with doing both and one of them is clearly very popular than the other. This sudden end makes more sense...