It's difficult to make sense of how trans people describe their sexuality, because they've redefined everything in terms of gender and also sometimes in terms of a split attraction model (romantic + sexual orientation nonsense). When they apply terms like gay or bisexual, it doesn't mean what we think it means. They refer to their actual sexual orientation as "genital preference".

With that in mind, I think I understand the phenomenon of how heterosexual men start identifying as bisexual after transitioning into transwomen. Their sexuality never actually changes. Make no mistake, they are still straight men. But they are straight men who try to live a life defined by female stereotypes. One of those stereotypes is that women should show sexual interest in men. Their female identities are validated when they are seen as sexual objects by other straight men. Adopting a bisexual identity and showing a reciprocal interest in men allows them to achieve that validation. They aren't actually bisexuals, but rather pseudobisexuals whose homosexuality is just another performative addition to their female costume.