First, calling a trans woman a man is misgendering, which is transphobic even if unintentionally.
I don't care if a rapist is offended by being called a man. They are male, and if they are also a rapist then they don't deserve people tripping over themselves to lie in order to be kind.
Secondly, it is not accurate and precise enough. A trans woman is not a man, but she is not just any woman, either. Accuracy and precision are important for good record keeping, for research purposes, and for correctly informing the public.
A trans woman is male. Yes accuracy is important which is why it is imperative they are recorded as male not female in order for accurate stats about male violence.
Thus, a transgender rapist should be recorded as such in police and cps records, and should be referred to in press releases as per Stonewall guidelines (transgender woman, then trans woman for subsequent mentions).
I do actually agree that police and CPS records should record the person as Male but with the acquired gender of woman, that way we would be able to get accurate stats about how many trans gendered sex criminals there are rather than them being conflated into female reporting.
If there is any doubt, I suggest a default of assuming a suspect is trans, to avoid giving offence. Stonewall guidelines make it clear that trans is an umbrella covering many things, including men who only very occasionally cross-dress, for whatever purpose.
Why does anyone care about giving offence in this situation? Why should anyone be compelled to lie about facts in order to keep a sex criminal happy?
It would help if all press releases included an explainer box about what a trans woman is, given the surprising number of people who don't seem to know.
Unfortunately Stonewall and many in the wider community keep changing what the description of commonly used words is so they have caused any communication misunderstandings. Up until a few years ago everyone knew what a Woman and what a Man was, now it's anyone who feels like one in that moment which has made it all a bit more complicated. It's pretty safe to assume that anyone who has used their penis to sexually assault another person is a man though.
This approach may make the use of 'cisgender' (eg for other suspects) unavoidable, which I know some will dislike. But avoiding ambiguity about what is really happening seems more of a priority at the moment.
So you expect everyone to acquiesce to the term cisgendered to appease the <1% of the population who have decided their gender doesn't match their sex, and actually to appease the even smaller % of that really small % who happen to be committing sex crimes. No. I will not accept the term cis.