Why is the tab not reopening?

Question

Angular

My code:

{{ selectedIndex }}

<mat-tab-group
  (selectedIndexChange)="selectedIndexChange($event)"
  animationDuration="0ms"
  [selectedIndex]="selectedIndex"
>
</mat-tab-group>

Why if you click open like this:

this.selectedIndex = 1;

Then the tab will open. Then if you go back to the first one there and click again - it does not come off