Bisexuals do seem to get ignored. I've run into people who dismiss the orientation as either being straight people who are experimenting, or gay people who are still in denial, or they shrug bisexuals off as people who can, and probably will, end up in hetero relationships, so they've got what amounts to straight privilege.
I don't agree with their assessment.
I do wonder if the under representation of bisexuality in fiction is because there's a tendency to focus on one relationship in a given story, and if it's with someone of the same gender, readers assume the character is 100% gay/lesbian, and if it's with the opposite gender, readers tend to assume the character is 100% straight? Explaining about the character's past relationships or attractions to other people, of course, is possible, but if the person isn't a pov character, or if there's no focus on their backstory, then it might not come out at all.
Of course, it's much more complex than that, because even if one is monogamously involved with someone of the opposite gender, that person would still have the attractions they have to your own gender too, and that's part of who they are (or vice versa). But there's such a tendency to paint people in fiction as if their "true love" is the only person they're even remotely attracted to or think about sexually, which is pretty unrealistic for someone of any orientation.
My own novel I'm shopping has a pov character who is gay. He's had some affairs with women in the past, but his dominant romantic and sexual attraction is towards his own gender. He's not the plot-driving protagonist in the first book, though in the sequel he becomes more important as a character who drives part of the lot, and he ends up in a long term relationship with a man who is bisexual.
One novel (started a while back, so there are some ways the situation is portrayed that might be less than ideal) where the main characters are bisexual is Lynn Flewelling's Nightrunner books. One of the two men in the central relationship seems to have a strong preference for his own gender, but occasionally has had dalliances with women. The other two seemed to be more frankly bisexual, as in he maybe even leaned a bit more towards women overall, but he fell for the other man and ends up in a monogamous relationship with him.
Some of MZB's stuff has bisexual female characters as well.