title: Angular material tab group problem on nested tab group (no default tab displayed)
body: <p>I'm facing the following problem with mat-tab-group nested inside a mat-tab-group. The nested group in the first tab of the parent group is selecting a default tab and showing the underline in the first tab, but the group nested in the third tab of the parent group, it's not showing the underline in any tab.</p>
  
  <p>I've attached the link of the demo with the problem: <a href="https://stackblitz.com/edit/angular-material-tabs-problem" rel="nofollow noreferrer">https://stackblitz.com/edit/angular-material-tabs-problem</a></p>
  
  <p>Any help? Thanks in advance!</p>