This is related to the configuration of the Drupal module Services at /admin/structure/services/list/oauth2_login_provider/server By default all types of request and response formats are enabled, and if the HTTP request does not specify a response format, XML is used by default. Either you can restrict this on the server, or make the client specify that they want JSON. In one of my JavaScript example I do it like this: B-Translator.github.io/api-examples-js/examples/oauth2/get_user_profile.js at master · B-Translator/B-Translator.github.io · GitHub
Apparently it also works if you append the extension .json to the URL (you discovered this). As long as it works, it is fine.