I think it's because the comic really didn't do a great job actually portraying their feelings or their developed feelings for each other. It just feels random and out of nowhere since all previous interactions before the 'grand confrontation' (ch. 75) were portrayed like friendly, naive happenings between friends or failed flirting attempts hitting deaf ears. There were, I think, 2 or 3 moments in which she blushed and looked like she had feelings for him, but every other time, there was literally nothing going on except just basic smiling and laughing as she did with everybody else. No change whatsoever.
That's why the romanc epart feels so boring to me at least; there is no effort in it.
I mean, she emotes more in comedic situations than she does in the love/flirting panels, so that's really not a surprise. And even the sad/negative situations have more emotion to them than the supposed romance. :/
I hope the novel is better in this aspect. But I have no time to check.
I just binged all of it and why is the romance with the duke actually the most boring part of this all