Immigration- Some countries will deny you the ability to immigrate if you are diagnosed at autistic.

Courts- If you have to deal with the legal system, there is the potential for a diagnosis to be used against you.

Police- Police are notoriously ableist, and if they know you're autistic, it could put you in more danger than you'd be in otherwise.

Infantilization- The general public is very poorly informed about autism. Many people in the general public think of autistic people as being severely cognitively disabled. Even many people in medical professions, who would have access to your diagnosis information through your medical records, are very poorly informed and might treat you like a liar or as though you are incapable of making your own decisions.

Workplace discrimination- While you would not need to disclose that you are autistic at work, if your workplace found out, they could use that information to discriminate against you. Yes, it's illegal and unethical, but workplaces do illegal and unethical things all the time. Also, it's possible for workplaces to find legal ways to discriminate against people, and if they know about your diagnosis, they could try to discriminate against you for that. Also, if potential workplaces find out about your diagnosis through other means, like social media, they could use that information to decide not to hire you. Yes, that's illegal, but it's very difficult to prove.