Getting a Custom Icon to Save in Icon Widget

Hi friends,

I am adding a custom icon set to the Icon widget via filter hook, per this guide:
https://siteorigin.com/docs/widgets-bundle/form-building/icons-and-fonts/

Everything seems to work up until the point of saving. After hitting the save button, the icon disappears from the “Choose Icon” button, and the selection is not saved (see screenshot below). The default set of icons all seem to have no problem being saved.

View post on imgur.com

Have I missed something? Is there another action or filter I need to pass data to before saving? My filter hook code is attached below, and also the plugin code itself is on GitHub (https://github.com/ridgekuhn/so-widgets-icon-fap).

Could you help me with stack tracing the save function for debugging? ie, where in the so-widgets-bundle is this form data prepared and passed to the database?

function rjk_sow_icon_fontawesomepro_filter( $icon_families ) {
  /** @var string $icon_meta_uri Path to the Font Awesome Pro icon metadata generated by parser.js */
  $icon_meta_uri = plugin_dir_url( __FILE__ ) . 'inc/fap-icons.json';
  /** @var string $style_uri Path to the Font Awesome Pro stylesheet */
  $style_uri = plugin_dir_url( __FILE__ ) . 'css/all.css';
  // Cache busting
  $style_uri .= '?ver=' . filemtime( plugin_dir_path(__FILE__) . $font . '/style.css' );
  /** @var array $styles Set up icon Styles list for SO Icon widget form */
  // [key] is rendered as a class name by siteorigin_widget_get_icon()
  $styles = array(
    'fab' => __( 'Pro - Brands', 'so-widgets-bundle' ),
    'fad' => __( 'Pro - Duotone', 'so-widgets-bundle' ),
    'fal' => __( 'Pro - Light', 'so-widgets-bundle' ),
    'far' => __( 'Pro - Regular', 'so-widgets-bundle' ),
    'fas' => __( 'Pro - Solid', 'so-widgets-bundle' )
  );
  /** @var array $icons Icon metadata generated by parser.js */
  $icons = rjk_sow_icon_fontawesomepro_import( $icon_meta_uri );
  // Register the icon set with SO Icon widget
  $icon_families['fontawesomepro'] = array(
      'name' => __( 'Font Awesome Pro', 'so-widgets-bundle' ),
      'style_uri' => $style_uri,
      'icons' => $icons
  );
  // Register style list with SO Icon widget
  $icon_families['fontawesomepro']['styles'] = $styles;
  return $icon_families;
}
add_filter( 'siteorigin_widgets_icon_families', 'rjk_sow_icon_fontawesomepro_filter' );