Maybe I'm being "too woke" but I'm sort of having a hard time reading how the author wants to interpret Kokuyou given the sequel having them constantly lament not being becoming a real girl and the dialogue in this but they keep drawing Kokuyou on twitter with the male sign? Might just be a cultural difference, I feel like even among trans groups I've seen language be more loose with calling trans women "former men" and vice versa, and I've literally seen posts from trans women in Japan referring to themselves as such in their own post. Chii's autobiography is "Hanayome wa Motodanshi" (The bride is a former man) despite being a literal autobiography about transitioning.
Basically I've noticed that "trans women = women" and vice-versa doesn't exist in the JP sphere even among LGBT. Always makes it difficult for me to judge stuff like this.
I've also wondered about this.
"Trans woman equals former man" certainly does seem like a cultural-societal thing. Either it's just not seen as rude
or being non-conformative in such a way entitles society to be as casually rude to you as they wish to be. In any case, I just chalk things like that up to "it's a cultural difference™" and then try not to think about it too deeply.
It probably does suck for Western trans manga fans: here's a tremendous source of media that features trans characters that also constantly invalidates them in these innocuous ways, even if that media is otherwise well-intentioned towards trans people.
In the case of this manga specifically, there does seem to be a heavy focus on how being gender non-conforming would negatively affect one's life (that conflict is actually why I find it interesting). I've been thinking of it as Kokuyou not being very aware of the Western conception of trans identity and having internalized "I'm not a real girl, I'm just cute and I like cute things, I'm just doing this to be with my friend," etc. It'd be interesting for the author to explore something like that, but it's also possible that Vae isn't even aware of it themselves.