初始化值为1的BehaviorSubject为何无法赋值给number类型？

场景代码 #

PlayerTurnService 服务实现 #

import { Injectable } from '@angular/core';
import { BehaviorSubject } from 'rxjs';

@Injectable({
  providedIn: 'root',
})
export class PlayerTurnService {
  private playerTurnState$ = new BehaviorSubject<number>(1);

  nextPlayerTurn() {
    this.playerTurnState$.next(this.playerTurnState$.value === 1 ? 2 : 1);
  }

  getCurrentPlayerTurn() {
    return this.playerTurnState$.asObservable();
  }
}

父组件模板绑定 #

<app-player-label [currentPlayerTurn]="playerTurn$ | async"></app-player-label>

子组件 PlayerTurnLabelComponent #

@Component({
  selector: 'app-player-label',
  template: `
    <p>Player {{playerTurn}} turn </p>
  `,
  standalone: true,
  styles: [],
  changeDetection: ChangeDetectionStrategy.OnPush,
})
export class PlayerTurnLabelComponent {
  
  @Input() playerTurn!: number;
}

报错信息 #

Type 'number | null' is not assignable to type 'number'.

临时修复方式 #

将子组件的输入属性类型改为number | null：

@Input() playerTurn!: number | null;

疑问 #

BehaviorSubject已经初始化了值1，且后续只会在1和2之间切换，永远不会出现null值，同时也给输入属性加了非空断言!，为什么async管道仍然会推断出number | null的类型？是不是对BehaviorSubject的初始化机制有误解？

内容的提问来源于stack exchange，提问作者onTheInternet