Someone deliberately deadnaming a trans person can 100% happen.

A trans person who knows how being deadnamed feels deliberately deadnaming a trans person usually doesn't. I won't say never, but most trans people would NEVER deadname someone deliberately if for no other reason than it opens them up to getting deadnamed.

Transphobia in books about trans issues is fine. I'm not saying there's any issue with Alicia having an issue with Leo being trans. I'm not saying there's any issue in anything in Almost Perfect (another book about trans teens) even though the trans character gets beat the fuck up.

This issue is that only a cis writer would have a TRANS character deliberately deadname another trans character.