I believe its because Lee muhan is a stranger to the current MC. So Lee Muhan never manifested to him, the Mc, previously before. Like have a full conversation or get to know him and earn his trust. We know that the Mc was chosen by the old man and that the other two enjoy his company so he the Mc got to know just three not the lee muhan that was basically a doormant fourth personality. He, lee Muhan was never activley there from the start. We had like little cameos here or there, but nothi g to solid for us to say consistent. The MC had never had any previous interaction, but since the current three personalities are now in danger of disappearing and morphing the MC has to listen to the original muhan's story to understand how to save his three boyfriends.
Wait let me clarify cause I think I'm lost in the plot. Lee Muhan was the vessel and the original body of the 3 Mugos? The 3 Mugos are the Gods(?) that manifested from what seems the bad doings of the family? Then why the present mc hates lee Muhan? Who's telling the backstory?