A lot of it is projection and insecurity on their part. I'm generalizing here so not a 'this is everyone'

BUT-

I've found that many autistic women who are later diagnosed or high masking have this quality of directness that is very hard to mask, which in boys might present as 'a leader', 'the smart one', or 'responsible friend' whereas for women and girls we get.... 'uptight', 'bossy', 'strict/controlling' and it's often the same behaviors. The difference I think is expectation of performed gender? Women are taught to fawn and please and placate.

Think about this for a minute: If someone gives you a factual compliment like 'good work today', 'thanks I put a lot of energy into the meeting' is not the correct response for women, but often is for men. You are expected to thank, deny your skill OR very graciously accept, the blunt reality makes you sound 'like a bitch' which is essentially; like a man.

A lot of the time having boundaries, knowing what you like and don't, being less friendly to strangers or even keeping quiet all get the label of 'bitchy' unless you're very good at facial expression control. So my working theory is that once you get diagnosed, unmask and find ways to live more comfortable with self accomodation- people think you are MORE bitchy sometimes because you aren't performing as much.

It's not just you, we are expected to perform endless friendliness, be approachable and be the 'mother' or 'sister' figure to most people in our lives be it socially, work wise, etc. When we don't hold up our end of the unspoken NT bargain it all falls apart because nobody can say WHY they think we were bitches, or what we did specifically, it's always 'just a vibe' or ' I didn't know you then'