Responses
Your not missing anything. It's going to be explained later.
My guess is "bye" in original text was not used between friends in everyday conversation but more like a "good-bye", as in "see you never"? The original meaning is likely lost in translation.
Korean doesn't have the same distinction that Japanese has between "see you later" (mattane, jaane etc) and "goodbye" (sayonara). Hangyul used 안녕, which is used between friends.
Is it just me or does the text just seem like Hangyul was just hiding something from sangjae and didn't want him to worry so he just sent that text to say "I'm fine, just have fun with your friends." It doesn't sound like Hangyul was saying to end the friendship. Am I missing something here? How did it get misunderstood to ending the friendship?