If you're referring to the "Redirect URIs" setting under the "OAuth 2" section of an app's page on the App Console, that controls which OAuth 2 redirect URI(s) are allowed to be used for that particular app.
That does not directly control which redirect URI is used at any particular point though. The actual redirect URI to use for a given instance of the authorization flow is set by the "redirect_uri" parameter on /oauth2/authorize. If the requested "redirect_uri" value is not in the "Redirect URIs" list, it will not be allowed.
If you're not seeing the redirect URI you expect being used, check what value you're setting as the "redirect_uri" parameter. If you're seeing a redirect URI being allowed when you don't expect it to be allowed, make sure you're using the app key (in the "client_id" parameter on /oauth2/authorize) for the same app that you're looking at on the App Console.