I think he started to have feelings but too stubborn to admit it
I believe it'll be revealed later that Nanjo started to think about mokyeon differently after ch 20.
Cause the way he interact with him after that changed ever so slightly.
And I agree with what you said. Though it's just a theory, but I think that when mokyeon called nanjo to see if he was okay or not and Nanjo rudely asked him if he was seeing suhwa again and told him to keep his end of the deal, he actually said it because he already had somewhat interest towards mokyeon already and felt a bit cheated on thinking mokyeon still sees suhwa. There were a few other scenes where Nanjo's interest(not feelings) were clearly visible. Rereading this after reading the whole authornote really made those parts so clear.
For the record, this is all just my theory.
that's what i was also thinking! also when that one time nanjo proposed ending that deal with mokyeon, in exchange of not seeing sunhwa, i'd like to believe it's because he's starting to catch feelings (or maybe interest, just like what u've said) and he wanted to continue that kinda relationship with mokyeon. this is also just my theory, i could be wrong. ˃
sunhwa just got stabbed in the back LOL anyway, why do i have this feeling that nanjo actually likes mokyeon? but that couldn't be the case....i am just being delusional ˶╹ꇴ╹˶