Being deliberately transphobic in translation is some losercore shit.
As a trans person myself, I understand how you feel. As you probably know, the subject in Japanese can be implicit, so the author doesn't actually write any pronouns in the narration. The way I think about this comic is that the author hasn't met a trans person before, which is how I justified to myself to not switch pronouns for her. So, that's on me. I'm sorry.
My reasoning was mostly just cowardice. I don't feel like MangaDex is a particularly accepting place, and I am very scared of mean comments or worse, especially as this was one of my first translations. However, I realise that is mostly just my anxiety. To apologise, I'll be changing the pronouns to 'her' in the final panel. I feel like that preserves the surprise while also acknowledging that the author now knows her well enough to know she uses 'atashi' and all that. Once again, I'm sorry for the transphobia.
Edit: and thanks for speaking up.