To be fair, it's not about bisexuals being cheaters, it's about the thrill of cheating itself + and picking the other female main character over a guy, usually.
Maybe there are some, but I don't really recall her doing "cheating lesbians", but I'd say that the reason for that isn't because she thinks or wants to portray bisexuals as morally depraved. It's probs because she fetishises the sort of cheating that's "feelsgood". As in, most readers would probs feel bad if the character was cheating on a girlfriend/wife tbh. Generally, to make readers root for cheating/not feel too morally uncomfortable with it, the partner that's cheated on must be someone they disapprove of. They may be abusive, they may be a cheater themself, they may be negligent, etc. Something is put into place to justify why "cheating is okay and although it feels wrong, it feels good and it's okay in this case". In yuri manga though, often just having the partner being male is enough for those who have a bias/much prefer women to disregard relationship morality as a factor of that equation.
That's simply what she likes to write. Rather than a jab at bisexuals, I feel like it makes sense, for what she wants to write, for her characters to end up being bi.
She likes bad girls, she likes dysfunctional relationships, she prefers having guys being the one cheated on.
In the end, it's her cup of tea and you can read whatever you want despite knowing that you dislike the author's tastes, but it doesn't make her "objectively" bad or anything if your main criticism is basically that you personally don't like what she likes to write. There's obviously an audience for her work. To be upset when you're not part of that audience is kinda weird imo.
