Symfony ManyToMany with existing entities - symfony

Question

When my requests hits the server, it should add 2 entries to the m:n table,
Core | Prop
1 | 10
1 | 11
Currently, it does add 2 new entities to the Prop table, but those do already exist, it just creates a new primary key and inserts a new item twice - and the assoc table holds then those 2 entities aligned with one core.
The Core Entity holds a m:n with Prop
/**
* #ORM\ManyToMany(targetEntity=Prop::class, inversedBy="cores", cascade="persist")
*/
private Collection $props;
Prop itself is a recursive entity:
class Prop extends BaseEntity
{
/**
* #ORM\Id
* #ORM\GeneratedValue
* #ORM\Column(type="integer")
*/
private int $id;
/**
* #ORM\ManyToOne(targetEntity=Prop::class, inversedBy="children")
* #ORM\JoinColumn(nullable=true)
*/
private ?Prop $parent;
/**
* #ORM\OneToMany(targetEntity=Prop::class, mappedBy="parent")
*/
private Collection $children;
/**
* #ORM\ManyToMany(targetEntity=Core::class, mappedBy="props")
*/
private Collection $cores;
Now this request gets sent:
"name": "a name",
"type": {
"id": 2,
"discr": "accr"
},
"props": [
{
"id": 10
},
{
"id": 11
}
],
When i debug the state before the persist() call, i do have 2 entities in here to be persisted with the correct id - yet it creates two new entities instead. There are no additional calls to it, request comes in - gets converted via the ParamConverter, followed by persist and flush.
Thank you.