GitLab Oauth2 login changes redirect_uri

I provide a web service and I would like let users to log in into it through their GitLab account. I am not using / do not plan to use GitLab in any other way.

I registered at GitLab (Free account), created an Application, generated Application ID / Secret, etc. See here:

When I try to log in, I see “The redirect URI included is not valid.” I am definitely sending them a valid redirect URI.

I also noticed, that at the GitLab login page, there is the redirect_uri changed into GitLab URL: