Huh??

No, it’s not a flashback! At the start they are at the event that Hyeon’s father dragged him along to (since he’s supposed to be ‘well’ enough now) and they start talking to Professor Park who he slept with in the previous chapter which obviously catches him off guard. Hyeon’s brother (half brother or step brother I forgot what he was lol) knew Hyeon’s mouth was sore and cut up - that part was a flashback of him drinking water some other time (I assume it got cut from sucking all that D lmao). So the evil brother purposely gave him a sparkling alcoholic drink (I guess that’s supposed to irritate it) in order to purposely embarrass him in front of his father and his father’s important friends. As Hyeon’s on his best behaviour trying to be how his dad wants and also distracted by the professor he slept with not that long ago, he accepts the drink without thinking and takes a sip which irritates his mouth (that’s why the SFX says “STING”. He then accidentally dropped the glass because it hurt his cuts and it shattered which gave him flashbacks to the trauma of when his mum fell down the staircase and died (we don’t know the full story of that yet tho). Assumably that accident involved broken glass and so when the wineglass shatters he has an intense flashback and disassociates from reality for a moment.

When he comes to, the professor he slept with has taken him into a private room (hence why they’re alone) because he wasn’t responding and needed to calm down after that. We as the audience don’t see this happen as we only see what Hyeon sees and he was spaced out for an unknown amount of time. So we don’t know how his brother and father reacted but we know Prof Park must’ve offered to pull him aside when he stopped responding and presumably his father let him go to prevent any more embarrassment. So that bits not a flashback, it’s technically a flash forward or time skip by a few minutes.

It’s definitely a bit confusing at first but I think that’s because we’re from Hyeon’s POV and he’s confused too! It helps if you have continuous pages setting on and remember that the flashbacks in this webtoon usually have a black background while the present time has white (other than moments of shock, blacking out etc).

Ik that was really long but hopefully clears it up!

oh and also to answer your other question I think the party is still going on in the other room, they're just in a study or private room with the door closed while Hyeon calms down.

Thank you SO much for your amazing explanation!! (づ￣ ³￣)づ I thought maybe some panels had been left out but turns out he dissociated!