Doctrine launches INSERT always instead of UPDATE for existing entities - php

Question

Let's say I have the following entities:
App\Entity\MainEntity:
/**
* #var object
*
* #ORM\OneToOne(targetEntity="App\Entity\DependentEntity", fetch="EAGER")
* #ORM\JoinColumn(name="DependentEntityType1FK", referencedColumnName="DependentEntityIDPK")
*/
private $dependentEntityType1;
/**
* #var object
*
* #ORM\OneToOne(targetEntity="App\Entity\DependentEntity", fetch="EAGER")
* #ORM\JoinColumn(name="DependentEntityType2FK", referencedColumnName="DependentEntityIDPK")
*/
private $dependentEntityType2;
Basically, one-directional 1:1 relationship from main entity to the same dependent entity using two different columns in the main entity table.
It doesn't matter, whether I use fetch="EAGER" or normal lazy loading through Doctrine proxy classes, when I do something like this:
$mainEntity = $this->mainEntityRepository->find(74);
$mainEntity->setDependentEntityType1($this->dependentEntityRepository->find(35));
$this->mainEntityRepository->saveTest($mainEntity);
where ::saveTest() is:
public function saveTest(MainEntity $mainEntity) {
$this->_em->persist($mainEntity->getDependentEntityType1());
$this->_em->merge($mainEntity);
$this->_em->flush();
}
it always tries to INSERT a new dependent entity to the table, even though I never made any changes (and even if I made them, it should have been UPDATE! for it)
The question is: why does Doctrine decide this dependent entity is a new one if I did $this->dependentEntityRepository->find(35) , so loaded an existing one?
I tried fetch="EAGER" thinking that spl_object_hash might return different hashes for a Proxy class instance and the actual DependantEntity one, but it doesn't matter, the DependantEntity is for some reason always considered as "new".
UPDATE: here is the code of ::setDependentEntityType1()
public function setDependentEntityType1(DependentEntity $dependentEntity) : void {
$this->dependentEntity = $dependentEntity;
}