Hey guys,
I had the same problem; all my custom widgets were missing from the front-end after updating WP (with gutenberg disabled) but they were still showing up fine in the dashboard
After some trial and error I managed to get them working — In the widget class, the update function needed to return an array, and then I needed to re-add the widgets and save them again in the dashboard.
public function form( $instance ) {
}
public function update( $new_instance, $old_instance ) {
$instance = array();
return $instance;
}