spoliers~ please read on your own discretion as I will explain FULLY by full I mean FULL. I saw so much people asking so I'll be the spoiler fairy starting from today kekw (copied my reply to here)
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 instinctively 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). And Han Yoojin wasn't invited at all because he's the biggest variable when it comes to tl1 and tl2.
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 (quoted from Han Yoojin's thoughts in the webnovel)
But obviously another stupid reason is because well... Hyj is his beloved... Item! He doesn't need a physical invite at all so he could just barge in there lol... Stupid shj trying to make hyj jealous... ╮( ̄▽ ̄)╭








This action will have consequences...
I'm scared to see the next incoming chapters ^^;
What's gonna happen?