Create_before_destroy icw depends_on resource list

Terraform
1

Conceptually I’ve got the following:

resource a a {
  foreach [ some-key-value-map]
  lifecycle {
    create_before_destroy = true
  }
}

resource b b {
  depends_on [a.a]
  foreach = [ some-other-key-value-map ]
}

Now when I rename a key in some-key-value-map a resource is created and destroyed. So far so good.
However, a resource in b.b is implicitly dependent on this, however I consistently refer to the renamed key in this resource in b.b.

Without create_before_destroy set to true, this fails, as the apply execution is as follows:

1a. create a.a[newkey] // these are executed parallel
1b. destroy a.a[oldkey] // these are executed parallel
2 update b.b[...]
…

This fails as the old resource is still referenced in the not-yet-updated b.b[...].
So far this is all expected.

However, I would’ve expected that introducing the create_before_destroy would’ve shown the following order of exection:

  1. create a.a[newkey]
  2. update b.b[...]
  3. destroy a.a[oldkey]

I expect this because the depends_on is on the whole of a.a. The aforementioned plus the explanantion in terraform/docs/destroying.md at main · hashicorp/terraform · GitHub expected me to have the desired execution order stated above where the destroy of a.a[oldkey] would be the last step.

Can someone explain me why the expected execution order is not happening for me?