Looping through custom fields in SAEF

I am using the SAEF to edit an entry.

Within the {custom_fields} tag I check for the field name( I added the sane field_name ) to the list of variables, however the although I can output {field_name_actual} and see the proper result, I cannot use it in the {if} tag, as it doesn’t exist.

Does anyone know why this happens and what the correct way to automate generating populated form fields is?