Only works if view is built off content

I'm pretty sure this is working as expected.

Any of the following fields are valid candidates:

Given that, You are more than welcome to add support to suite your needs. To get it working, you just need to implement a couple of hooks. Specifically you could do something like what's mentioned in views_natural_sort.api.php and do

function custom_views_natural_sort_supported_properties_alter(array &$supported_properties) {
  // Allow the vns_field value to be naturally sorted.
  $supported_properties['block_content']['type'] = [
    'base_table' => 'block_content_field_data',
    'schema_field' => 'type',
  ];
}

This should allow to override that behavior.