I don't know if this will have any relevance any more as the original post is quite old. However here goes. I also noticed that querying the API for custom fields does not return them unless they are specified in the return fields. But the field that is listed in the API explorer in not the actual field name, I could not get the fields to return using their "title names" but they do when using the custom_xx field name.

I also noticed that querying the API for custom fields does not return them unless they are specified in the return fields. But the field that is listed in the API explorer in not the actual field name, I could not get the fields to return using their "title names" but they do when using the custom_xx field name.