In order to debug what is happening on the PHP side you need to do something like write the log file https://www.php.net/manual/en/function.error-log.php.
I still don’t think I have a clear picture of when the field is created. The information you’ve given does not tell me if the code is run during the ajax request. Is it done in your functions.php file, or some other file that is loaded every time WP initializes?
