It is not too clear to me. Send links, logins etc and we can check that how it works.
I will try to explain it with the screen shots bellow:
1- I create a new Custom field with the name " Surface "
2- I added a new listing and filled the new custom field with " Test Text"
3- As you can see in item page, the new custom field doesn't appear
4- in the listing edit page the Customd field "Surface" is empty as well
5- But when i fill the surface custom field in Edit page
6- The result in item page
Conclusion: The custom field appears only when editing the post. But when you add a new post, in this case the custom field remain empty
I hope this will help