WordPress Apply filter from Class isn’t working

So I’m trying to apply a filter to a variable that I have in a class and its not doing anything. I’ve added the variable in another class thats on a separate file and it works there, but when using it on this one it doesn’t. Here is how I created it and how I’m calling it. Thanks for the help

class WPFrontendForm{
   public function __construct($labels = true, $display_required = true){
    add_filter( 'wpfrontendform_required_fields', array($this, 'wpfrontend_required') );
}
       // Create filter for required fields.
   public function wpfrontend_required(){
   $rfields = array();

    foreach ($this->fields as $field => $value) {
        if( $value['required'] == true ){
            $rfields[] .= $value['id'];
        }
    }
    return $rfields;
}

And I call it inside of my other class here:

class WPFrontendProcess {
  public function validate_form(){
   $required = apply_filters( 'wpfrontendform_required_fields', $required );
  }