In some senses, “phases” are a thing. Aside from sexually-fluid and genderfluid people for whom going though various phases is fundamental to who they are, there are people who “try on” various identities for awhile until they find one which reasonably fits who they are, and LGBTQIA+ people who go back to identifying as (read: pretending to be) cishet because they want a career and a family and a good relationship with their parents and such… though thankfully that’s becoming a lot less common. And even bi people who pretend to be gay/les for awhile because they have ties to a G or L community which is biphobic. All of those could be considered “phases” in some sense, due to the fact that their outward identity changes for awhile.
But even so, a phase can only be identified as one in retrospect. It’s very rude to assume that whatever someone identifies as right now is merely a temporary quirk.