What happened to them in the past?
Spoiler because I don’t know if u have read this or not
Well jaehan basically ruined heegyeons career by setting him up. In the manga it shows how jaehun set up drugs in heegyeons house during a party to basically frame him as someone who takes drugs which obviously in Korea is frowned upon , this created a very big scandal that destroyed his acting career and blacklisted him. Heegyeon found out what he did. The reason why he framed heegyeon was stupid but basically seeing him on screen made his heart tighten
Jaehan is the reason why Heegyeom got jailed in the past so they ignoring each other for a while but not any more because they had s--.
It's obvious that they are inlove with each other but too scared to admit it because of the past still lingering with them.