这是因为在第一次更改时，触发更改侦听器的变化检测尚未启动。解决此问题的方法是使用Zone.run()方法显式触发变化检测。示例代码如下：

import { Component, NgZone } from '@angular/core';
import { FormArray, FormBuilder } from '@angular/forms';

@Component({
  selector: 'app-form-array',
  template: `
    

      

        

          
        
      
    
  `,
})
export class FormArrayComponent {
  form: FormGroup;

  get items(): FormArray {
    return this.form.get('items') as FormArray;
  }

  constructor(private fb: FormBuilder, private zone: NgZone) {}

  ngOnInit() {
    this.form = this.fb.group({
      items: this.fb.array(['Item 1', 'Item 2']),
    });

    this.items.valueChanges.subscribe((_) => {
      console.log('Value changed!');
    });

    this.zone.runOutsideAngular(() => {
      this.items.controls[0].setValue('New item', { emitEvent: true });
    });

    this.zone.run(() => {});
  }
}