Strange behavior using indexBy collection

[scion4581]

Example:
2 entities

class Promotion  
@ORM\OneToMany(targetEntity="Product", mappedBy="promotion", indexBy="articleId", cascade={"persist"})
@var Product[] $products
private $products;

public function removeProduct(Product $product): void
{
    $this->products->removeElement($product);
    $product->setPromotion(null);
}

class Product
//unique field
private $articleId;
@ORM\ManyToOne(targetEntity="Promotion", inversedBy="products", cascade={"persist"})
@ORM\JoinColumn(onDelete="SET NULL")
private $promotion;

There are two options to fetch Promotion and related Products

1. Custom DQL with JOIN (result: 1 query to the database)

$this->createQueryBuilder('promotion')
        ->select('promotion','products')
        ->leftJoin('promotion.products', 'products')

2. Usual EntityManager findBy (result: 1 query for Promotion and second later while accessing product collection)

$em->getRepository(Promotion::class)->findOneBy(['id' => 11111]);

The problem appears when I am trying to remove concrete Product from Promotion product collection

$product = $em->getRepository(Product::class)->find(2);
$promotion->removeProduct($product);
$em->flush();

Expected result:

UPDATE product SET promotion_id = null WHERE id = 2

But with OPTION 1 nothing happens, Doctrine remove the association only using OPTION 2, or if I remove indexBy="articleId" option

It looks like Doctrine does not track as usual collection indexed by a custom field...

I will be glad for any help