Angular 9 mat-自动完成功能不能与switchmap操作符一起正常工作

<input type="text" [formControl]="myControl" [matAutocomplete]="auto">
<mat-autocomplete #auto="matAutocomplete">
  <mat-option *ngFor="let option of filteredOptions | async" [value]="option">
    {{ option }}
  </mat-option>
</mat-autocomplete>

import { Component } from '@angular/core';
import { FormControl } from '@angular/forms';
import { Observable } from 'rxjs';
import { switchMap, startWith, map } from 'rxjs/operators';

@Component({
  selector: 'app-autocomplete',
  templateUrl: './autocomplete.component.html',
  styleUrls: ['./autocomplete.component.css']
})
export class AutocompleteComponent {
  myControl = new FormControl();
  options: string[] = ['Option 1', 'Option 2', 'Option 3'];
  filteredOptions: Observable<string[]>;

  constructor() {
    this.filteredOptions = this.myControl.valueChanges.pipe(
      startWith(''),
      switchMap(value => this.filterOptions(value))
    );
  }

  filterOptions(value: string): Observable<string[]> {
    const filterValue = value.toLowerCase();
    return of(this.options.filter(option => option.toLowerCase().includes(filterValue)));
  }
}