Doctrine OneToOne relation bidirectional - symfony

Question

I'm facing an issue concerning OneToOne relation between some Entities.
The subtlety is that the owner side can be multiple classes (all children of the same class).
Here is the summary
I've got Process which have several children entity classes (let's take ProcessPassport, ProcessIdentityCard and ProcessCadaster for example)
Some of my children entities need a relation with another entity TaxStamp, but not all of them. So I cannot put this association in the Process main entity.
Instead I created a trait TaxStampTrait, containing my relation mapping.
But this does not work properly :
I cannot define multiple possibilities for the association on the TaxStamp entity
Doctrine tells me there are error in my annotations for the relation, because I refer to the Process (legit since only some entities use the TaxStampTrait)
Question: what should I do to make it all work the right way?
Here is the gist with a summary of the code and classes: https://gist.github.com/bastos71/8e15f69ebecf5e97dc75187d130fe109
<?php
/**
* #ORM\Entity(repositoryClass="ProcessRepository")
* #ORM\InheritanceType("JOINED")
* #ORM\DiscriminatorColumn(name="discr", type="string")
*/
class Process
{
// ...
}
<?php
/**
* #ORM\Entity(repositoryClass="ProcessCadasterRepository")
*/
class ProcessCadaster extends Process implements EntityWithTaxStampInterface
{
use TaxStampTrait;
// ...
}
<?php
/**
* #ORM\Entity(repositoryClass="ProcessIdentityCardRepository")
*/
class ProcessIdentityCard extends Process implements EntityWithTaxStampInterface
{
use TaxStampTrait;
// ...
}
<?php
/**
* #ORM\Entity(repositoryClass="ProcessPassportRepository")
*/
class ProcessPassport extends Process implements EntityWithTaxStampInterface
{
use TaxStampTrait;
// ...
}
<?php
/**
* #ORM\Entity(repositoryClass="TaxStampRepository")
*/
class TaxStamp
{
/**
* #ORM\OneToOne(targetEntity="Advercity\AdminBundle\Entity\Process", mappedBy="taxStamp")
*/
private $process;
// ...
}
<?php
trait TaxStampTrait
{
/**
* #ORM\OneToOne(targetEntity="TaxStamp", cascade={"persist"})
* #ORM\JoinColumn(name="tax_stamp_id", referencedColumnName="id")
*/
private $taxStamp;
// ...
}