How to fix cascade persist?

Question

Doctrine ORM

I have two entities

class Orders{

   /**
     * @ORM\OneToMany(targetEntity=Positions::class, mappedBy="ord",cascade={ "remove", 
"persist"})
     */
    private $positions;
}

class Positions{
 /**
     * @ORM\ManyToOne(targetEntity=Orders::class, inversedBy="positions")
     * @ORM\JoinColumn(name="ord",referencedColumnName="guid",onDelete="CASCADE")
     */
    private $ord;
}

when trying to delete an order

class OrdersRepository{
public function delOrder(){
   ....
   $this->em->remove($order);
   $this->em->flush();
}
}

, an error occurs:

A new entity was found through the relationship 'App\Entity\Positions#ord' that was not configured to cascade persist operations for
entity: App\Entity\[email protected] To solve this issue: Either explicitly call EntityManager#persist() on this unknown entity or configure cascade persist
this association in the mapping for example @ManyToOne(..,cascade={"persist"}). If you cannot find out which entity causes the problem implement '
App\Entity\Orders#__toString()' to get a clue.

I have it, but it doesn’t work ...

As soon as I put it in the order deletion function:

class OrdersRepository{
public function delOrder(){
   ....
   $this->em->remove($order);
   $this->em->flush();
   $this->em->clear(); //?????? верно ли, почему  помогло?
}
}

the error disappears, and why? honestly, I spied the answer there , but did not understand why, and is it true?