Contact form - Custom fields are not sending content
Drupal 8 will handle custom fields on contact forms 'out-the-box', they will be rendered in the message body by contact module. If custom fields don't appear in message body, someone messed up.

In this particular case, attribute 'name' of 'input' html tags were modified, and $_POST came with a wrong schema. $_POST['field'] instead of $_POST['field'][0]['value'].

Here shows the hint:
"message":[{"value":"Any message"}] - Correct (field[0]['value'])
"field_customa":[] - Wrong (field[])