Honestly didn't see that coming that his ex was the other mc's boyfriend (from the author's other book)
Rereading this a second time and realized that the seme from this book, is a childhood friend of the uke of the other book. And the ex of the uke of this book, as shown, is the present boyfriend of the uke in the other book. It's as if they got an exchange or smth. Dunno if the sht I'm writing is confusing but you'll get it if you read the other book
Ore no Oniku-chan