a.jsx

// Build a hook to register a callback to run every frame (subscribe(...))
// to copy some state into our component if some condition is met
function useStateCheck() {
  const {
    state,
    subscribe
  } = useContext(context);
  const [_, setState] = useState(state);

  useEffect(() => {
    const unsubscribe = subscribe((newState) => {
      if (someCondition) {
        // For some reason, calling this setState here (used in component below) causes the
        // component to unmount and remount.
        setState(newState);
      }
    );
    return () => unsubscribe();
  }, []);

  return state;
}
            
function MyComponent() {
    const myState = useStateCheck();
    
    useEffect(() => () => console.log("This gets logged when the hook sets state, and I don't know why"));

    return <div>
      ...
    </div>;
}