See the issue and the test case with original fix in https://github.com/facebook/react/pull/14802.

We'll need to fix it properly, see https://github.com/facebook/react/pull/14802#issuecomment-463277621.
switch (action.type) {
        case 'increment':
          return {count: state.count + 1};
        case 'decrement':
          return {count: state.count - 1};
      }
    }
    function SomeComponent(props) {
      const [state] = React.useReducer(reducer, props, p => ({
        count: p.initialCount,
      }));
      return (
        <div>
          <p>
            The counter is at: <span>{state.count.toString()}</span>
          </p>
        </div>