Widgets disappeared from "Available Widgets" but still work

I’m not sure exactly when this happened but several of my custom widgets no longer appear as “Available Widgets”. They do still show up on the sidebars though and are working.

The widgets are all custom and are included in the theme functions file like this:

include_once(ABSPATH.'/wp-content/widgets/subscribe.php');
  include_once(ABSPATH.'/wp-content/widgets/events.php');
  include_once(ABSPATH.'/wp-content/widgets/search.php');

and all of the widgets are registered in their individual php files like this:

register_sidebar_widget(array("Ads", 'widgets'), 'widget_ads');

I have no idea where to even start looking…