Hha
Because if my memory serves me right and what the last chap has shown me, it seems SuminxSungji's relationship is quite shallow compare to SejuxSumin's in the past?
I always want to put the "perception" disclaimer when it comes to this, but it's kind of hard not to see what you have seen in these recent chapters. Especially during their morning talks.
In the beginning of their relationship, they had their moments, but as the relationship develops, it becomes apparent whether it has the substance to last or not.
Overall, I have noticed a lot of repetition in the content they talk about. Like in this last chapter and in chapter 79 regarding Sungji getting free coffee.
Like what hlama mentioned too:
Something along the line of at first I thought I would be happy just looking at you, then I realized how good it felt to touch you, to kiss you.
That was said in chapter 84 most recently, chapter 64 as well, and the topic of Sungji loving Sumin's face has been mentioned many times, if I recall. How deep is that exactly? To me, it's not the deepest it could be, honestly.
The sex scene in chapter 85 where Sungji asked Sumin what she loved about her, the first thing that came to mind for Sumin was her beauty. And the last chapter shows more of Sumin's thoughts towards Sungji.
Sungji herself is realizing that Seju's relationship with Sumin is hard to beat. They let their guards down with each other. As many have said before, Sumin and Sungji's relationship has been very... cautious.