Demi Lovato was previously diagnosed with Bipolar Disorder. They have since rejected the diagnoses from what I've heard. Perhaps they have Borderline Personality Disorder.

It's a common feature of these disorders, especially borderline, to cause the patient to experience what's called "Identity Disturbance". That means basic facts about your identity, like who you are, what you like, and what matters to you change rapidly, leaving you with very little to call yourself a "self" about. Gender identity can plausibly be one of those things.

I have bipolar disorder. The way Demi Lovato describes gender as a flux of masculine and feminine energy makes a lot of sense to me. Without having heard them say it, it seems exactly like what I say. So much about me is fluid and ever changing. It took a disruptive life change involving a misguided religious conversion (fortunately mostly private) leading to a mild depression to really get some sense of what makes me "me". I actually thought about and wrote down 13 things that seem to apply to myself across all seasons of life (and year, month, and day). The fact is, gender isn't one of them. I don't experience gender extremes, and most of the time am "neutral", but sometimes I can feel masculine and look back and think, "Ew, I was too femme then, what was I thinking?" and other times feel femme and look back and feel grossed out by that masculine loser. There just is no fact of the matter. It changes.

For Demi Lovato's "gender journey", they should probably try to be as treatment adherent as they can be to find some coherence about their identity, and maybe in fact they will have a binary gender. But I'm very treatment adherent myself and it just doesn't happen with me. There's no pill that makes you a permanent, fixed self. It might be that fluidity is the norm for some people.