Female as an adjective is perfectly fine.
A female patient, a female politician, a female customer, etc. That's the best way to refer to those.
What's bad is using 'female' as a noun: "A female. "
In general, you just don't use adjectives-as-nouns to refer to people. You don't call someone "a gay", "a black", or "a Chinese". That is offensive, and "a female" has the same kind of feel.
(there are exceptions to the above: you can call someone 'an American' or 'A German", but not "A French". I don't understand why - if you can't feel your way, best just avoid it)
Now, you could get around it by calling someone "a female person" - except that we already have a word for "female person", and that's "woman". And to go out of your way to avoid saying "woman" makes you sound like some kind of incel weirdo, and you don't want that.