Why does simultaneous saving of entities on the same page not work?

Question

Doctrine ORM

Can you please tell
me why saving entities in one repository function does not work (sometimes it works, sometimes it doesn't)?
I'm in pain and I don't understand why.

class OrdersRepository(){
            public function fff()
                {
                    $positions = $this->positionsOrdersRepository->getPositions($date, $step);
                    foreach ($positions as $position) {
                        if (...){
                            $position->setTitle(3);
                            $this->positionsOrdersRepository->setSave();

                            $position->getOrder()->setTitl("0");
                            $this->setSave();
                        }
            
            }
            
            }
    }