Why content disappears when switching tabs?

Question

Angular

I use mat-tab as a non-smart wrapper, and when switching between them, the content part disappears. But, at the first initialization, the content part is present. Why does content disappear when switching?
non-smart wrapper code:

<mat-tab-group mat-align-tabs="start" (selectedTabChange)="onTabChanged($event)" [selectedIndex]="selectIndex">
  <ng-container *ngFor="let reportLabel of labels">
    <mat-tab label="{{reportLabel}}">
      <ng-content></ng-content>
    </mat-tab>
  </ng-container>
</mat-tab-group>

@Input() labels: any;
  @Input() selectIndex: any;

  @Output() tabChanged: EventEmitter<any> = new EventEmitter<any>();
  onTabChanged(tabChangeEvent: MatTabChangeEvent): any {
    this.tabChanged.emit(tabChangeEvent);
  }

main component code:

<app-tabs-group (tabChanged)="tabChanged($event)" [selectIndex]="0" [labels]="reportLabels">
        <div class="wrap">
          123123
        </div>
      </app-tabs-group>

reportLabels = ['1 год', '6 месяцев', '3 месяца', 'месяц'];