I think that's the continuation of season 2 last episode
i was so confused too !!! i know that yeon actually told him that HE LOVES HIM for the first time (if i remember correctly) when they were making love in the toilet but he never proposed him so i don't get it why kihoon said that and even wrote that
Chop off my previous answer... maybe it's an Easter egg...??? For the upcoming new season!!! So excited(≧∀≦)
I think him writing it down on paper and putting it in the omegaverse book was just him journaling the nice dream he had in this sidestory ლ(´ڡ`ლ) since it was implied that yeon was proposing in the dream and wanting to get married, probably inspired by the book's ending which is why he hides it there afterwards (?) When he writes it down, he's in the same bed as the first episode of the sidestory so I'm guessing it's the morning of!
(I do the same sometimes, too, when a dream is interesting yet I also forget about what I write a while after lol)
Can someone explain the last panel of kihoon writing “yeon proposed to me” im a little confused