In #42403 checking for uniqueness of certain collection keys was enabled. Method UniqueValidator::reduceElementKeys removes all keys which are not specified.
Problem is that this happens before normalization, which in my opinion is not great because that method accepts array argument and if i have some object (DTO), TypeError will be thrown.

Example:

class ParentDTO
{
    /**
     * @var ChildDTO[]
     */
    #[Assert\Unique(
        normalizer: [ChildDTO::class, 'normalize']
        fields: 'id'
    )]
    public array $children;
}

class ChildDTO
{
    public string $id;
    public string $name;

    public static function normalize(self $obj): array
    {
        return [
            'id' => $obj->id,
            'name' => $obj->name
        ];
    }
}

Because normalization will happen after reduceElementKeys this will be thrown:
TypeError: Symfony\Component\Validator\Constraints\UniqueValidator::reduceElementKeys(): Argument #2 ($element) must be of type array, ...\ChildDTO given, called in .../UniqueValidator.php on line 48

If $element = $normalizer($element); is executed before reduceElementKeys it would enable using Assert\Unique with array of objects when correctly normalized