[v7.0-beta.7] firstValueFrom does not unsubscribe synchronously

[spalger]

Bug Report

Probably working as intended, I'm just looking for clarification

Current Behavior
When firstValueFrom() subscribes to an observable it behaves subtly different than source.pipe(first()).toPromise() because it doesn't unsubscribe from source until after it is done synchronously emitting events.

Reproduction

• REPL or Repo link: https://stackblitz.com/edit/rxjs-zsh7mp?file=index.ts

import { Observable, firstValueFrom } from 'rxjs'; 

const values = [1,2,3]
const source = new Observable(subscriber => {
  while (!subscriber.closed && values.length) {
    subscriber.next(values.shift()!)
  }
  
  subscriber.complete();
});

firstValueFrom(source).then(console.log);
console.log(values) // expect values = [2, 3]

Expected behavior
I expect that subscriber.closed would be true because the consumer is only expecting a single value.

Environment

• RxJS version: 7.0.0-beta.7