Where is the redirect_uri in the authorize request uri being set?

I'm getting a 400 Bad Request error related to the redirect_uri attribute in the authorize request URI not matching to what is set in the app settings. I have reviewed the resolution to this error in the knowledge base (https://support.okta.com/help/s/article/The-redirect-uri-parameter-must-be-an-absolute-URI) and understand that under normal circumstances, the solution would be to update the Sign-In redirect URI in the app settings, however. the issue that I have is the redirect_uri value in the authorize request uri is incorrect while the Sign-in redirect URI in the app settings is correct.

I'm using the custom-login example code for PHP and I can see the where the redirect_uri value is being set, however, the value that I've set it for does not appear in the authorize request URI. So, my question is... how or where does Okta obtain the redirect_uri value when it constructs the authorize request URI after a successful login?