psalm: [MismatchingDocblockReturnType] I don't get how this works

class Foo {
  /** @var \stdClass[]|\ArrayObject */
  public $bar;
  
  /**
   * @return \stdClass[]|\ArrayObject
   */
  public function getBar(): \ArrayObject
  {
    return $this->bar;
  }
}

In real use case, this is a problem for Doctrine Collections, where docblock is specified like this

/**
 * @return RoomItem[]|Collection|Selectable
 */
public function getRoomItems(): Collection
{
    return $this->roomItems;
}

ERROR: MismatchingDocblockReturnType - src/Entity/Item.php:75:8 - Docblock has incorrect return type ‘Doctrine\Common\Collections\Collection<mixed, App\Entity\RoomItem>|Doctrine\Common\Collections\Selectable’, should be ‘Doctrine\Common\Collections\Collection’

If I change it to

/**
 * @return RoomItem[]|Collection
 */
public function getRoomItems(): Collection
{
    return $this->roomItems;
}

error is no longer shown, even though it’s pretty same like original example code