Array does not keep previous elements after being updated - php

Question

Job entity has a property called candidates which is an array of Users and it is not mapped or related to any other entity.On given route we fetch specific Job object by id.We update our candidates array by assigning the user who we get from the current session.It turns out that the user is being saved, but on each call we lose those users which had been saved before.
Why does that happen?
/**
* #Route("/job/apply/{id}", requirements={"id"="\d+"}, name="student_candidate", methods={"POST"})
*
* #param int $id
*
* #return JsonResponse
*/
public function apply(int $id)
{
$job = $this->getDoctrine()->getRepository(Job::class)->find($id);
$candidate = $this->getUser();
$job->addCandidate($candidate);
$this->getDoctrine()->getManager()->flush();
return new JsonResponse([
'status' => 'success',
'message' => 'some_success_message'
]);
}
class Job
{
/**
* #var int
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #var string
*
* #ORM\Column(name="title", type="string", length=255)
*/
private $title;
/**
* #var string
*
* #ORM\Column(name="content", type="text", length=65535)
*/
private $content;
/**
* #var User[]|ArrayCollection
*/
private $candidates;
public function __construct()
{
$this->candidates = new ArrayCollection();
}
/**
* #return User[]|ArrayCollection
*/
public function getCandidates()
{
return $this->candidates;
}
/**
* #param User $user
*
* #return Job
*/
public function addCandidate(User $user)
{
$this->candidates[] = $user;
return $this;
}
}