export class DestroyTest implements OnDestroy {
  ngOnDestroy() {  // <<<<< this method is NEVER called
    console.log('ngOnDestroy DestroyTest called');
  }
}

export class DestroyTest2 implements OnDestroy {
  ngOnDestroy() { // <<<<< this method IS called when the app ref is being destroyed
    console.log('ngOnDestroy DestroyTest2 called');
  }
}

export function createDestroyTest() {
  return new DestroyTest();
}

@NgModule({
  imports:      [ BrowserModule, FormsModule ],
  declarations: [ AppComponent, HelloComponent ],
  bootstrap:    [ AppComponent ],
  providers: [
    {provide: DestroyTest, useFactory: createDestroyTest},
    DestroyTest2
  ]
})
export class AppModule {
  constructor(
    destroyTest1: DestroyTest, destroyTest2: DestroyTest2) {}
}

Angular version: 6.0.0-beta.x