Mat-checkbox [checked]=“false” does not work

Trying to implement a checkbox where I can dynamically set the value (and have it appear in the box).

HTML

<mat-checkbox *ngIf="isReply()" class="col-2" formControlName="checkbox" [checked]="false"  >CheckBox off</mat-checkbox> 
<mat-checkbox *ngIf="!isReply()" class="col-2" formControlName="checkbox"  >CheckBox</mat-checkbox> 

materials.module.ts

addCheckbox() {
      this.checkboxForm = this.fb.group({
        'CheckBox':true,

main.module.ts

isReply(): boolean { 
  return: true;
}

There is a radio button that toggles isReply() off and on.

When I toggle isReply() on, I can see the CheckBox label change from "CheckBox" to "CheckBox off" but the checkbox status (visibly) does not change.

I can apply other logic which responds to the checkbox being off, even though the checkbox is still visibly checked (true). This changes the value of the checkbox to false, even though the checkbox is still visibly checked (true).

When I click on the checkbox (clear the visible box) the value toggles and the response is as expected, the value of checkbox is now true, even though the box is not checked.