Problem statement
The preview page in the Auth0 dashboard shows correctly when enabling the background image for the New Universal Login, but the same image is not visible on the login page when testing with a log in attempt.
Solution
The customizations from the dashboard are overwritten if there is a template set with the Management API when a login flow happens with a custom domain.
There is a sample for how to add a background image in the HTML template using the Management API. This sample provides a good starting point.