ParentalManyToManyField symmetrical=False

[mightym]

I'm using on my BlogPost model the ParentalManyToManyField on the Model itself via self.
And I just realized that the field causes related Pages to update other Pages.

Similar to what would happen when using ManyToManyField with symmetrical=True

class BlogPost(Page):
    related_posts = ParentalManyToManyField(
        "self", blank=True, related_name="related_blog_posts"
    )

from blog.models import PostPage

one = PostPage.objects.get(id=1)
two = PostPage.objects.get(id=2)
three  = PostPage.objects.get(id=3)

one.related_posts.all()
Out[6]: <PageQuerySet []>

two.related_posts.all()
Out[7]: <PageQuerySet []>

one.related_posts.add(one)
one.related_posts.all()
Out[9]: [<PostPage: Post 1>]

two.related_posts.add(one)
two.related_posts.all()
Out[12]: [<PostPage: Post 1>]

two.related_posts.add(three)

two.related_posts.all()
Out[16]: [<PostPage: Post 1>, <PostPage: Post 3>]

one.save()
five.save()

one.related_posts.all()
Out[18]: <PageQuerySet [<PostPage: Post 1>, <PostPage: Post 2>]>

two.related_posts.all()
Out[19]: <PageQuerySet [<PostPage: Post 1>, <PostPage: Post 3>]>

So I assumed I have to use symmetrical=False. But this seems to have no effect. Am I' missing something or misunderstood the concept of ParentalManyToManyField?