From a developer:

In the /classes/class-fl-custom-modules-example-loader.php file, on lines 30-31, you have this:

// Enqueue custom field assets.
add_action( 'init', CLASS . '::enqueue_field_assets' );

However, when you get down to the enqueue_field_assets function (lines 55-57, you have this:

if ( ! FLBuilderModel::is_builder_active() ) {
return;
}

However, is_builder_active() isn't available yet, since that function was loaded on init, so the enqueue_field_assets function will always return at that point and never enqueue the field assets that are placed a couple lines later.