Well seeing as how that gamer member(sorry I forgot his nameTT) said "how could Jun Hyung do that to Jaeyoung Hyung"after Jaeyoung told him about what happened, I think he knows who drugged him, and he also had that flashback dream one time so guess he does. Also I don't really know but I think Seungmin has a suspicion or even knows who drugged Jaeyoung since he specifically looked up Jun and found all that erased data in the internet. And when meeting those Kwon dude on the bar he said something like "is it a coincidence?" With Jun having the same last name, even if he isn't sure I think he already have put two and two together!
Happy reading!

ok idk if iām just dumb but im a bit confused with plot. so they both have the stage name happy and happy is also the name of the drug. jaeyoung was drugged but is he aware he was drugged? and does seungmin already know who drugged him? i may have to go reread