Angular 9: set default value with async pipe doesn't update form validity

Question

I have a FormGroup, let's call it groupForm, that contains a control which is a group of radio buttons:

this.groupForm = this.formBuilder.group({
   radioGroup: ['', Validators.required]
});

I have a BehaviorSubject in a service, which I'd like to use to set the default value of that group. I'd like to use an async pipe to do it. So in my template I do something like this:

<mat-radio-group formControlName='radioGroup' [value]="selection$ | async"> 
... radio buttons here ...
</mat-radio-group>

Then, in the same template I am using the validity of the groupForm to enable a button.

The problem is, although in the UI the correct radio button appears selected, the form is still invalid and the button appears disabled. I tried the following workaround in code:

this.selection$ = this.selection$.pipe(
    tap(value => this.groupForm.get('radioGroup').setValue(value);
)

and it worked. But I am wondering if I am missing something and I can avoid using this workaround.

Thank you in advance.