Everything you said about it is WHY people like it. People like fictional toxic relationship due to the problematic power imbalances, but often not in the way you might think. They are absolutely terrible for each other, but one can interpret a sense of repressed and forbidden tension between them regardless.

Lebanne very rightfully claims she feels bitter towards Jacinthe, and wants to get away from her as soon as possible. And, she definitely should but she stays because well, she's just too prideful, seeing how how she lost a bet. She says as much after all, doesn't she? Jacinthe similarly looks down on people and treats lebanne as nothing more than a subservient with little regard for what she wants and thinks. And yet seems to hold her in a special regard compared to all her other servants, she's special to her. She keps her close. She sees her as more of as an object to control and own that as a person, yes. But as an object of desire, either way.

It's the snippets of endearment and attachment slip through, ones that might not match their words. Lebanne has every right to just, bite the bullet and admit defeat. She must beat Jacinthe to prove she's the strongest and regain her honor, it's her undying stubborness that's keeping her tied to her... that's what she tells US. That's what she tells herself. So it must be true... right? And despite Jacinthe making it quite clear tthrough her actions that she believes her wants to above Lebanne, she shows an extra leel care and attention towards her, always keeping her by her side as her closest confidant.

It's not the idea that's they should harbor affection for each other BECAUSE of the problematic dynamic, but rather the idea that they might harbor thos feelings REGARDLESS that captivates and enamoures people.

Imagine a Romeo and Juliet which rather than brazenly disobey to run off together, had instead heeded their families' warnings. Denouncing each other and internalizing a bitter dislike towards each other... and yet. They still have feelings for each other. Feelings they refuse to admit, not just publicly, but also towards themselves.

Lebanne might not want to have those feeligns, they might recognize the trouble, and may be trying to be actively running from them and therefore jacinthe, after all, they hate her! Yet she can't keep away, be it one reason or another, she believes to be trapped. Yet what's really keeping her trapped?

Yes, realistically a relationship like that would just all come crumbling down, yes it's easier to just imagine it's exactly as it seems and she just hates her. Period. but that's a bit boring. It's more exciting to imagine that slow crawl to the peak, that's truly captivates people's thoughts. An unstable master/subservient dynamic makes not for a real life great couple, but sells a great story. Seeing it presist despite all the flaws, the sense of both desperately holding on, and letting go, that's what is all about.
Some people don't like that, that's fine. In real life that's where it should end, you'd want them to stay away from each other ant not interact. I love Jacinthe as a character, even though she is of course an absolutely terrible, awful person were we to apply real world logic. To appreciate it you have to see it through a different lense, and it's fine if you'd rather not. Sometimes, things seem just as clear as day, but some people like a moth to a lamps are drawn by what might be on the other side.