It's here!!! The Birthday arc, bruh feel like this a love confession scene straight from a shoujo manga" while reading lmao
spoliers~ please read on your own discretion as I will explain FULLY by full I mean FULL.
In the webtoon, it was shown that Sung Hyunjae seems to be sensitive about the regression (such as imagining Han Yoojin's face to be older~ stuff like that).
Sung Hyunjae at the time felt like preparing and just the birthday party was something he had already experienced (which he did), hence the bad mood he's been having. Because he felt so bored of doing the same thing again.
I guess this explaination will be shown more probably the next chapter, such as Bak Yerim's invitation being late (Since she was just an A rank in the first timeline) and Myungwoo's being even later (since he was presumably dead/just a simple f rank).
Han Yoojin wasn't invited because Sung Hyunjae felt a sense of dissonance, and he was probably gonna exclude Han Yoojin and compare what he was feeling (Han Yoojin's thoughts in the webnovel)








HYJ: I'll give you the birthday gift you've been looking forward to.
SYH: Yourself?