laravel morphToMany relationship create() method don't save relatedPivotKey expectly

It's a many-to-many relationship. [Attachment] can be used for user's avatar or comment detail attachment image. table structure:

CommentDetail
    id - integer
    cid - integer
    content - text
User
    id - integer
    name - string
Attachment
    id - integer
    key - string
AttachmentRelationship
    id - integer
    target_type - string ('user_avatar' or 'comment_detail')
    target_id - integer (User -> id or CommentDetail -> cid)
    attachment_key - string (Attachment -> key)

first, in AppServiceProvider, I had custom morphMap:

Relation::morphMap([
    'comment_detail' => CommentDetail::class,
    'user_avatar' => User::class,
]);

then in CommentDetail model, define attachment() method to get all attachments:

public function attachments()
{
    return $this->morphToMany(
        Attachment::class,
        "target",
        "attachment_relationships", 
        "target_id",
        "attachment_key",
        "cid",
        "key"
    );
}

now the problem comes. when I create a attachment through commentDetail like this:

$commentDetail = CommentDetail::findOrFail(4);
$attachment = $commentDetail->attachments()->create([
    'key' => (string)\Uuid::uuid4(),
]);

it will create a [Attachment] record:

id    key
10    968e22b8-e4fb-4743-bf08-8ac9cd8ecd56

and a [AttachmentRelationship] record:

id   target_type    target_id    attachment_key
1    comment_detail    7            10

my question: why the [AttachmentRelationship] record 'attachment_key' field's value is not the [Attachment] record 'key' field's value = 968e22b8-e4fb-4743-bf08-8ac9cd8ecd56 (now it's 'id' field's value = 10) ?

via Chebli Mohamed