This. If you watch today's episode, they literally avoided each other. Or more on Somin avoided JK almost all the episode. But while pillow fighting game, they chatted just fine. Nothing change about their interaction. So, they, on purpose decided not to interact with each other in front of camera.
Also, during opening, YJS, JSJ, Haha talked about her leaving RM, JK said almost nothing. JK is among the oldest members, it is his place to say something (teasing, encouraging etc) about Somin's departure, but JK just mostly stayed quiet. You can compare to how he acted when members teased LKS's leaving RM.
As for the MAYO song, the lyric literally reminded me of Kookmin in late 2019/early 2020. JK who was sweet and cheesy to Somin. JK who stared at Somin with soft gaze. JK who was so happy interacting with Somin. Of course all these made her heart flutter yet also made her confused. Did he really likes her or it is just her delusion. Maybe that's why in the song, if it is just her illusion, she asked him to not to be good to her. She didn't want to raise her hope only to get hurt later.
But, from Shim's part, we know that the man indeed likes her and he won't hide his feeling anymore. I saw the significant change in Kookmin interactions after the vague phase. JK vowed to take care of Somin in ep 500. Somin make a hand gesture to JK during Be Mine song (Infinite) in a bus ride. The sudden intense skinships between Kookmin after she came back from her break.