If the user is already logged in, url redirects from the login page using the returnto query parameter don't work.

For instance: https://test.musicbrainz.org/login?returnto=https%3A%2F%2Ftest.musicbrainz.org%2Fnew-oauth2%2Fauthorize

If you are already logged in and open the url, you will be redirected to your user profile instead.