Hey you know what's super funny about the idea of "good bi rep"?
For a character to be canonically bi you have to make sure and establish that they're attracted to multiple genders. Not all mediums allow you to get inside every character's head or show what they're thinking. Flirting can be read ambiguously, and god forbid they flirt with a character who's not into them and be read as pushy or predatory. So it can be super handy to just mention an ex or two! But you better not mention too many exes because that would make them a slutty bisexual which is (checks notes) bad, and you definitely better be careful about making them poly, because that might make them, uh... greedy. Oh, and those exes? They better be perfectly amiable breakups with no conflict or drama, because it's bad to represent queer people in toxic or abusive relationships (especially queer women! very bad), and you definitely can't have them have lost a partner if the partner was queer because that's "bury your gays..." You should probably also eliminate all trauma from their backstory, just to be safe. You should probably also make sure they're not involved in crime, deception, or anything of the sort, because that would make them "deviant" and a stereotype.
But don't worry! Once you've carefully crafted your nice, monogamous, experienced-but-not-too-experienced Lawful Good bi character, you will be rewarded with your audience deeming them "boring" and quickly passing them over for other characters. :)
This is honestly an issue with a lot of good representation in media. The problem is that bigots tend to make up a lot of stereotypes that are often contradictory, but always framed negatively. And this is by design, of course, because the point is to be able to point at any random human of those categories and be able to pick out a trait they—like all people—are statistically bound to have and say, “See? This confirms my bias!” So it’s almost impossible to construct a character that’s in any way interesting or realistic without rubbing against a stereotype some assholes have decided to paint an entire group with. That said, you can definitely have representation that is clearly coming from someone with a lot of bigoted stereotypes in their head, and every creator is going to have cultural biases, regardless. Still, it’s better shake off the chilling effect that the fear of being grouped in here causes and get more representation out in the world, because only through diversity of representation in each category do those stereotypes start to lose their power.
Good addition and this brings up something critical that I didn't say explicitly in the original post. Some stereotypes are things that are objectively negative, such as bisexuals being manipulative or deceitful people (see: Maureen from Rent). That still doesn't mean, in my opinion, that fiction should never include characters with those traits, only that it's all the more important to make them complex and well-rounded characters (and to have a variety of representation so those characters aren't all we have).
But also, a lot of stereotypes are things that aren't actually bad, they're just framed negatively, usually because of other social stigmas. The negative perception of bisexuals being overly sexual, or having had multiple partners and relationships in their life, or being polyamorous--this is an intersection of biphobia with slut-shaming, amatonormativity, general sex negativity, etc. Are any of these things actually bad, for a bisexual person or otherwise? No! They're fine! It's fine to like sex and to have lots of sex. It's fine to have many relationships in your life. It's fine to be poly. None of these things are actually negative traits, and in fact in some cases they are things that could themselves use more representation in media: sex positivity, polyamory, relationships that don't adhere to a heteronormative, amatonormative standard.
So what we have are character traits that aren't even negative traits, alone or in combination, but they become negative stereotypes because we are used to seeing them framed negatively. Again, this doesn't apply to all stereotypes of marginalized groups, and also it's fine and important for characters to have actual negative traits. But I think it's worth taking a minute when we're looking at one to say, hang on, is this actually a bad thing or is it just stigmatized? And how does that affect how we want to approach it in fiction, how we want to frame this character?