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.
function rerenderReducer<S, I, A>(
  reducer: (S, A) => S,
  initialArg: I,
  init?: I => S,
): [S, Dispatch<A>] {
  const hook = updateWorkInProgressHook();
  const queue = hook.queue;
  invariant(
    queue !== null,
    'Should have a queue. This is likely a bug in React. Please file an issue.',
  );
  queue.lastRenderedReducer = reducer;
  
  
  const dispatch: Dispatch<A> = (queue.dispatch: any);
  const lastRenderPhaseUpdate = queue.pending;
  let newState = hook.memoizedState;
  if (lastRenderPhaseUpdate !== null) {
    
    queue.pending = null;
    const firstRenderPhaseUpdate = lastRenderPhaseUpdate.next;
    let update = firstRenderPhaseUpdate;
    do {
      
      
      
      const action = update.action;
      newState = reducer(newState, action);
      update = update.next;
    } while (update !== firstRenderPhaseUpdate);