A wordpress site keeps editing wp-content files after migration

You may have an issue with parent/child theme overwriting like Jacob Peattie said in the comments.

Another possibility is that your hosting provider is changing something during that restore process. Check your wp-config.php settings to make sure they haven’t changed, and check your Settings > General tab in wp-admin to make sure your domain hasn’t changed. The WordPress Address (URL) and the Site Address (URL). I know of a few hosting providers who try to convert the site’s domain when migrating and restoring backups, and if you have a custom domain setup and this changes from its original value, then your site doesn’t know how to find the theme content that is hard linked and located in wp-content. Any relative links would still work if the base domain changed, but any hard links would have been broken.
(This might be hard to check if you don’t have access to any root files)