The seventh parameter passed to add_submenu_page()

I was able to track this down to the culprit function add_theme_page(). There

There was an additional parameter, per the codex for add_theme_page() that needed to be removed. Removing that seemed to help.

function fivehundred_register_admin_menu() {
  add_theme_page(
    '500 Settings',
    '500 Settings',
    'manage_options',
    'theme-settings',
    'fivehundred_admin_menu',
    plugins_url( '/ignitiondeck/images/ignitiondeck-menu.png' )
  );
}

add_action(
  'admin_menu',
  'fivehundred_register_admin_menu'
);

Fixed code

function fivehundred_register_admin_menu() {
  add_theme_page(
    '500 Settings',
    '500 Settings',
    'manage_options',
    'theme-settings',
    'fivehundred_admin_menu'
  );
}

add_action(
  'admin_menu',
  'fivehundred_register_admin_menu'
);