Been a while since I last read the main story but from what I remembered, Hyunsoo being Hyunsoo, honest and a rule stickler, he audited his department and it revealed what his collegues were doing. IIRC, they were passing some data from their company to their rival (El Lune). And he's the whistleblower, he revealed it to their boss and that's why he was re-assigned and their department was dissolved.
idk if that's understandable lol

Can someone tell me what’s the deal with mc’s past colleague that appears in the earlier chapters? during their joint department event, the one with black hair. I don’t really understand his intentions. And what beef did he have with mc?