In Melissa and The Vicar by S.M. LaViolette, the heroine Melissa is bisexual and the only lovers she has had in the past few years before meeting the hero have women.
As it’s a historical romance, during the course of the book the only lover she has is the hero- but as the owner of a brothel at one point she is going to take up clients again after she and the hero have gotten together (as she thinks it was a one time thing and he won’t be able to track her down).
In one of Jill Sorenson’s books Dirty Eleven the heroine engages in a threesome with her boyfriend and another woman and I think the threesome part is all or mostly f/f although the heroine doesn’t identify as bisexual.
Other than those I can’t think of any historical or contemporary romance off the top of my head that feature a bisexual heroine although I am sure there are others out there.