Sandbox: https://codesandbox.io/s/cranky-kapitsa-op3jh

Repro: type "a" and then after a second "b"

Expected: Eventually you'll see "ab" in all fields. Pending "..." indicator goes away.

Actual: You'll keep seeing "a" in low pri field. Pending "..." indicator is stuck.
const getVersion = source._getVersion;
  const version = getVersion(source._source);
  const dispatcher = ReactCurrentDispatcher.current;
  
  let [currentSnapshot, setSnapshot] = dispatcher.useState(() =>
    readFromUnsubcribedMutableSource(root, source, getSnapshot),
  );
  let snapshot = currentSnapshot;
  
  
  const stateHook = ((workInProgressHook: any): Hook);
  const memoizedState = ((hook.memoizedState: any): MutableSourceMemoizedState<
    Source,
    Snapshot,
  >);
  const refs = memoizedState.refs;
  const prevGetSnapshot = refs.getSnapshot;
  const prevSource = memoizedState.source;
  const prevSubscribe = memoizedState.subscribe;
  const fiber = currentlyRenderingFiber;
  hook.memoizedState = ({
    refs,
    source,
    subscribe,
  }: MutableSourceMemoizedState<Source, Snapshot>);
  
  dispatcher.useEffect(() => {
    refs.getSnapshot = getSnapshot;