Function to query DB nested in a fetch loop returns false even the query is OK on test

Question

can someone explafind out WHY i have a query: SELECT something FROM MyTable

while ($stmt->fetch()) {
   // EVERYTHING I DO HERE IS FINE
   ...
   $ss['ss_id'] = $ss_id;

   //THIS IS THE CALL I AM ASKING ABOUT
   $ss['crew'] = $this->getStarshipTeam($ss_id); 
}

function  getStarshipTeam($starship_id) {
            //THIS QUERY WORKS IN MYSQL WORKBENCH TEST
            $q = "SELECT u.nome, u.cognome
                  FROM users u, user_props
                  WHERE user_props.starship_id=?
                  AND user_props.id_user=u.id_user
                  ORDER BY u.cognome ASC
                  ";

            $crew = [];
            // RETURNS $stmt = false when run in the code!!! :(
            if($stmt = $this->conn->prepare($q)) {
                $stmt->bind_param("i", $starship_id);
                $stmt->execute();
                $stmt->bind_result($nome, $cognome);
                while ($stmt->fetch()) {
                    array_push($crew, $nome." ".$cognome);
                }
            }
            else {
                die('error');
            }
            return $crew;
        }

basically it is a fetch loop which retrieves $ss_id; and calls the function to do the parallel query...

any hint why this happens?