I’m not even sure I get what’s bad about that. Are they saying it’s anti-trans or something? I doubt many transwomen would be offended by the word ‘female’ in that context. And if it’s for sexism, that’s not really the sexist part. So I’m very confused.

To be clear, there’s nothing wrong with using female as an adjective.

Calling someone a female customer / politician / patient / etc is perfectly fine and normal.

The problem comes when you use it as a noun.

Think of calling someone “a gay”, “a black” or “a Chinese”.

You just don’t use adjectives-as-nouns to refer to people, apart from a very limited set of exceptions.

Instead you talk about a gay, black or Chinese person.

The same goes for ‘female’.

And since there’s already a word for ‘female person’ - ie. ‘woman’ - then going out of your way to avoid it makes you sound like a raging incel.