Omg that is such a good theory maybe they did have a really big fight but I think sunwoo ( brown haired boy) was ultimately the one who ghosted the other boy
I have also thought abt that! I also feel they did communicate ineffectively which had a domino effect before the actual falling out. Sunwoo was probably the overwhelmed one and Haejin was unable to comprehend some things and kept questioning things in his mind. I see it that way bc so far, Sunwoo is being dishonest and his conscience is weighing on him (especially knowing that the prize from the event was supposed to go to Haejin’s father’s expenses if Haejin did won) plus he treats Haejin like a vase that he is afraid to shatter so I feel like all these crumbled feelings deep inside him that he is concealing will subtly burst during their fall out.
However, seeing how it is implied that Haejin is having a rough time (lacking academically, with his father still in the hospital, and working to earn for himself and for his family even just a tiny bit, being lonely), how its evident that Sunwoo is influencing how Haejin decides on things, and how its depicted that Haejin was summoned by the teacher in the recent chapter (im thinking its probably abt his grades and his father’s situation but he was not that down bc he has Sunwoo that time), I feel like “that mistake” Haejin is referring to might really be the plagiarism issue as u said bc it would strain relationship if it really was Haejin’s work, it is dramatic and big for writer lovers, but it doesn’t explain why Haejin told Sunwoo he gave up on them too easily (if its abt plagiarism solely, and it is likely to cause a break up, a sentence like ‘you gave up on us easily’ from someone who got plagiarized would mean he doesnt value plagiarizing more than he values Sunwoo) or the line where Sunwoo says “It’s not you, it’s me” (bc this suggest more). We know how much respect Sunwoo gives to Haejin’s writing and passion, Sunwoo also knows how much potential Haejin has judging by his talents as evidenced by the awards he previously won and would’ve won in spite of his difficult life. Seeing all this after winning an award which he won by plagiarising (and being a son of a known writer), it might have took a big chunk of his confidence and being with Haejin might have hurt.
Sorry thats so long.
my take on the reason of their fall out is that he plagiarised something that black haired (i tend to forget names im sorry) actually wrote? i honestly feel like it would be too far fetched if it wasn't something that black haired boy wrote bcs from the story's build up, their fall out was probably an INTENSE one.. like they probably fought and argued before the fall out happened.. they probably didn't go down the gradually losing contact due to ghosting or awkwardness route?? idk.. while i do acknowledge that writing is important to both the characters, i don’t think brown haired boy plagiarised something that someone random made (like i just feel that wasn't enough of a reason to cause such a big strain on their relationship, like sure, there would be a strain on their relationship after that but it wouldn't be as big and dramatic as this one??) it definitely has to be black haired boy or someone black haired boy knows??? idk, how about you guys?? what's your take on this one??