After children form value changed, parent form value is not updated

I have a nested forms. I used the patchValue to initialize the child form. Many people here may ask why I didn't set child form fields with default value while creating a new form, the answer is the child form need to be initialized from a REST API, but in sample code, I just use patchValue to demo my issue.

ngOnInit() {
    this.form = new FormGroup({
      input: new FormControl("", [Validators.required, Validators.maxLength(5)])
    });

    this.form.patchValue({
      input: "test"
    });

    this.form.valueChanges
      .pipe(untilDestroyed(this))
      .subscribe((value: string) => {
        this.onChangeFunc(value);
      });
  }

Please refer to my sample code: https://stackblitz.com/edit/angular-8-app-example-ktwtye

Problem: the child form value are initialized, (state is valid), but the parent form value is still null (state is invalid)

Any suggestion will be appreciated about : How to properly update parent form value after child form value is updated by patchValue or setValue ?