Repro case with a master build: https://codesandbox.io/s/stoic-mcnulty-dhygf?file=/src/App.js:668-703

Expected: we see content after a second.
Actual: fallback never resolves.

This happens in a sequence of:

1. Normal update
   - which suspends
2. Idle update
   - which also suspends
3. An unrelated normal update immediately followed by a ping (so they're batched)
   - here, we decide to stay on fallback, but should've shown the content

If you remove `_setVersion(v => v + 1);` on line 44 then the issue goes away.
expect(ReactNoop.getChildren()).toEqual([]);
    
    Scheduler.unstable_advanceTime(100);
    await advanceTimers(100);
    expect(Scheduler).toHaveYielded(['Promise resolved [A]']);
    
    
    expect(Scheduler).toFlushAndYield(['D']);
    expect(ReactNoop.getChildren()).toEqual([
      span('Loading...'),
      span('C'),
      span('D'),
    ]);
    
    await null;
    
    expect(Scheduler).toFlushAndYield(['Bar', 'A', 'B']);
    expect(ReactNoop.getChildren()).toEqual([
      span('A'),
      span('B'),
      span('C'),
      span('D'),
    ]);
  });
  it('suspends rendering and continues later', async () => {
    function Bar(props) {
      Scheduler.unstable_yieldValue('Bar');
      return props.children;
    }
    function Foo({renderBar}) {