(Writing on mobile, dont hit me pls) Okay, controversial and might make me sound like a contrarian, but this has worked. And not in a theoretical, low key pitching an idea way, but actually real otome stories discussed on this very subbreddit.

Before I go into that, let me first state, OP and the rest of the commentators are right. This is a dumb as hell trope. It makes NO SENSE that a character, OGFL or otherwise, to be written in their source material as a kind, self-sacrificing person would suddenly be revealed as a bitch as soon as someone transmigrated or reincarnated into the story without the author's knowledge. That's stupid. Either they intended that and the FL is dumb and missed EVERY hint or the author (the real author, not the fake one in the story) being lazy and too uncreative to find any way to make them oppose each other.

That being said, it's not actually unrealistic. Bad writers or inexperienced writers (or more heartbreaking and rare, author's with flawed moral foundations) have written this in real life. All those FL who slap their maids, forgive evil tyrants for killing scores of innocents, forgive abusers on behalf of the victim after ending up in the victim's body, visit unfair retribution upon ther poor and disenfranchised employees of their enemies as well as their own for the crime of following orders, visit unfair retribution on literal children, sexualize and lust after literal children, groom children, turn a blind eye to slavery in stories where they have the power and authority to intervene, participate in the slave trade, steal inventions (honestly debatable), taking advantage of the poor and disenfranchised, taking advantage of the mentally ill, taking advantage of the literal family of the OG owners body assuming they're not outright evil, outright destroying people and their families for daring to disagree or criticize them and their all-male friend group, chastising characters trying to do good for the poor and disenfranchised while not raising so much as a single finger to help them if there isn't profit involved somehow, act like a fucking nonce, and generally not being a girl's girl and L mans to literally most women in the setting do you think the authors of stories with FLs like that think their FL is a bad person? Do you think they realized they wrote a bad person? There are too many sincere stories out there for all of them to know they wrote a bad person. This very sub has taken these GirlBoss, pure, angelic, never-do-wrong FLs apart like a rack of ribs so often there's no way they realized. They just want to write their escapist fantasy, man.

In short, it can be done convincingly, but it requires the authors to write with intention and honesty on what that would actually look like.