She knew quite awhile ago frankly. As dense as she acts in front of Patrick, she's too clever fail to analyse herself.
Which is, by the way, presented slightly different in the novel version. I'll only talk about stuff that is "equal" in terms of spoilers with the manga version, but so far the manga has already diverged quite a bit. Not in plot per se, but certain characterizations and presentation.
She really is dense in the novel version. In that version, this revelation is less of her coming to that conclusion about Patrick through self-reflection, and more like her going "wtf?", vaguely when she really has to come out and publicly admit it. Which doesn't really match how observant she is of the various factions and political considerations in the story.
The manga has much more of a "yeah it bothers people, but I'll do it my way anyway," and "I didn't think of that because my values are so warped by my level" feel. She misses things but it's somewhat more natural and she's aware of the gap. The novel is much more....frankly she's an idiot, who Mary Sue's a sudden genius understanding of the situation when it's plot convenient.
I've seen some people complain that the heroine's love interests are cardboard cutouts, but trust me, they're far worse in the novel. And the Heroine joins them in that too. Some scenes have already changed somewhat....I suspect the manga is planning to diverge in plot as well, while fleshing out more characters that should be fleshed out. The monarchy was another one. Basically, the manga, while still fairly simple in plot and characters and slice of life (so far), is making some effort to improve on the original story. And boy, it needed it.
So I'm really interested where she goes with this self-realization. I'm not expecting the romance plotline between her and Patrick will change....but I'll bet I can't rely on the novel version to know exactly what comes next, and that's great.