i can assure you that joohyuk is NOT part of taehyung’s past!!! the moment he met him was the first time.
also, a breakdown that i feel like everyone needs to know is: it’s true that we never saw a reaction out of the two of them whenever jihoon’s name was dropped, but that was not because they didn’t care about him being the villain in both their lives, but as silly as it may sound—jihoon is a pretty common name in korea and they didn’t think they were talking about the same person and also!!!! in chapter 7, when taehyung told jeongguk a bit of his background, he did mention jihoon, but jeongguk had his utmost attention to what taehyung was saying because he wanted to understand him better, hence why they never really asked each other about who the fuck jihoon was for them. the surprise in taehyung’s face when hayoon went to pick aera up with joohyuk was because the name he always associated to hayoon’s partner was jihoon, so we could’ve had him putting two plus two together but honestly he too has a lot on his plate that hayoon’s partner wasn’t one of his main concerns, let’s put it that way (and although he was somehow part of jeongguk’s life too, why would he ask about jeongguk’s ex’s partner?) so yeah, i hope it explained a bit the situation and i didn’t make it more confusing <3