Why does my friend invalidate my bisexuality but not others' bisexuality?

There could be many reasons. Maybe they have internalized biphobia, so they're theorietically ok with bisexuality but they get mixed feelings when someone they're close to turns out to be bisexual. Or maybe they're acting like this because they're worried that living as a bisexual would make things hard for you. Whatever the reason, you should openly talk about it, tell them how it feels for you to be who you are and how it feels when they invalidate this side of your identity. You can encourage them to express their questions and doubts and answer them in order to help them understand you better - and also to help them better understand their feelings towards bisexuality, and overcome their internalized biphobia if needed.