(Source: https://developer.apple.com/documentation/authenticationservices/aswebauthenticationsession)

Unfortunately, Opera does not support AsWebAuthenticationSession, so every time another application needs to authenticate the user in a browser, Safari is opened instead of Opera (my default browser).

Applications that use are amongst others: DeepL, ChatGPT, and EduVPN.

Other browsers experienced the same issue, like Firefox and Chromium. Please check the link how it was resolved there.