Hi Tibor,
I guess if you use any custom field API like ACF or maybe PODS or something else, it is possible, that the custom field values are not yet present in the post_meta database, until they have been used at least once in a post. They are requested directly from the database via a query.
The values can be cached as well, as they are printed as the page content itself. Once the page/object cache is cleared, they should show up.