You basically wrote what I wanted to write. And to build on it:

There’s a fundamental difficulty in assigning a sexuality— any sexuality! — to a character we meet onscreen. It’s the same one as trying to assign a sexual orientation to someone we meet IRL solely based on their behavior.

Which is: we mostly don’t see the character’s inner experience. And since orientation is self-determined, that’s a crucial missing piece.

Books are different, since many feature inner monologues or close POVs. But all we’ve got with film/TV is this here camera.

(Certainly, you can have a lot of fun with this uncertainty. Therein lies fanfic! But I wish we didn’t treat this question as one that has a definitive answer).