It is intersting that you consider this an "interesting question". because this "interesting question" makes no sense at all in a belief system where trans men are men.
Mastectomy scars literally do mean they aren't men, using the original sex-based meaning of the word.
I understand some people want to use their own personal definitions of men and women (1), but that is a contested position not an accepted fact.
Women who believe some aspect of their personality is not compatible with womanhood and want to somehow transsubstaniate into men are still women.
It is an entirely female question that arises from the experience of being female but not wamting to be, and how that interacts with the social construnctions around the female body and female social presence. It is literally impossible to consider this question in the conext of trans men are men because it's starting in the wrong place and using the experience of the wrong people. And the fact you see the question as interesting shows that you also know this.
It would be much more honest, progressive and productive to find new names to label these new aspects of personality that people want recognised rather than bastardise and invalidate the existing lanaguage of sex by tying it to the age-old prejudices we have been trying to get away from.
But hey ho, trans people are fundamentally selfish in this respect, believing their need to redefine other people is more important that others' right to their own existence, their own rights and their own history as men or women.
(1) which as far as I can tell seem to boil down to "the same old sex-based definitions for everyone else, except I am allowed to be treated like I am that sex too. Don't ask me to explain how that doesn't invalidate the very group I am trying to join and undermine the justiifcation for the existence of the very resources I am trying to appropriate, how dare you question my existence! I have a right to define my own existence, and I have a right to define yours as well. You have no say in this because you do not experience the deep distress I do"