ell...I'm honestly glad Mi-Jin found out. They've been playing this game for too long. They both like each other but they never establish an official relationship. I know the author is doing this on purpose to add drama/spice to the series and it was needed at first. It gave room for JJ to save MJ from a car accident + rapist and they had their tender moments in the hospital and with the kiss.
The early drama was needed for JJ and MJ to develop history and affection for each other. At this point, they're just beating around the bush and not using the words "I like you." It's also not the fault of just JJ or MJ but both of them. JJ tried to establish a relationship with MJ early on and got turned down. He can push her out of the way of a speeding car and get beat up to protect MJ from a rapist but he can't say "I like you?" MJ keeps using the excuse that she's just helping him out of obligation as a friend but I bet she'll get angry at MC next chapter even though they're not in an sort of official relationship.
They both like each other for sure. They just never set any boundaries and end up getting angry or feeling guilty. With this chapter, they get another chance to officially establish that they like the other person but it'll probably be used for the sake of drama.
tldr They're not being honest with themselves and using JJ's condition to keep a status quo