UseReducer hook doesn't update state (Can't perform a React state update on an unmounted component)

Question

I'm trying to use useReducer instead of useState in a custom hook that loads the initial data from the API, and getting an error updating a state. (I use useReducer here for learning purposes).

The component fetches data firstly correctly, the error occurs when I update the state (book/edit/delete interview).

I left the previous useState code in the comments for better understanding.

import { useReducer, useEffect } from "react";
import axios from "axios";

const SET_DAY = "SET_DAY";
const SET_APPLICATION_DATA = "SET_APPLICATION_DATA";
const SET_INTERVIEW = "SET_INTERVIEW";

const reducer = (state, action) => {
  switch (action.type) {
    case SET_DAY:
      return { ...state, day: action.day }
    case SET_APPLICATION_DATA:
      return {
        ...state,
        days: action.days,
        appointments: action.appointments,
        interviewers: action.interviewers
      }
    case SET_INTERVIEW: {
      return { ...state, id: action.id, interview: action.interview }
    }
    default:
      throw new Error();
  }
}

export default function useApplicationData() {
  // const [state, setState] = useState({
  //   day: "Monday",
  //   days: [],
  //   appointments: {},
  //   interviewers: {}
  // });

  const initialState = {
    day: "Monday",
    days: [],
    appointments: {},
    interviewers: {}
  };
  
  const [state, dispatch] = useReducer(reducer, initialState);

  //updates the spots remaining when book/edit/cancel interview
  const updateSpots = (requestType) => {
    const days = state.days.map(day => {
      if(day.name === state.day) {
        if (requestType === 'bookInterview') {
          // return { ...day, spots: day.spots - 1 }
          return dispatch({ type: SET_DAY, spots: day.spots - 1 });
        }else {
          // return { ...day, spots: day.spots + 1 }
          return dispatch({ type: SET_DAY, spots: day.spots + 1 });
        }
      }
      // return { ...day };
      return dispatch({ type: SET_DAY, spots: day.spots });
    });
    return days;
  }

  //sets the current day data
  // const setDay = day => setState(prev => ({ ...prev, day }));
  const setDay = (day) => dispatch({ type: SET_DAY, day });

  //adds new interview data to database
  const bookInterview = (id, interview) => {
    const appointment = { ...state.appointments[id] };
    const bookOrEdit = appointment.interview ? 'edit' : 'book'; //defines the request type
    appointment.interview = { ...interview };
    const appointments = { ...state.appointments, [id]: appointment };
    
    let days = state.days;
    if (bookOrEdit === 'book') {
      days = updateSpots('bookInterview');
    } 

    return axios
      .put(`/api/appointments/${id}`, {interview})
      .then(() => {
        //setState({ ...state, appointments, days });  
        dispatch({ type: SET_INTERVIEW, id, interview });
    })
  };

 //deletes interview data from database
  const cancelInterview = (id) => {
    const appointment = {...state.appointments[id], interview: null};
    const appointments = {...state.appointments, [id]: appointment };
    const days = updateSpots();
    
    return axios
      .delete(`/api/appointments/${id}`)
      .then(() => {
        //setState({ ...state, appointments, days });
        dispatch({ type: SET_INTERVIEW, id, interview: null });
      })
  };
 
  useEffect(() => {
    let isMounted = false;
    Promise.all([
      axios.get('/api/days'),
      axios.get('/api/appointments'),
      axios.get('/api/interviewers')
    ])
    .then((all) => {
    //   setState(prev => ({
    //     ...prev,
    //     days: all[0].data,
    //     appointments: all[1].data,
    //     interviewers: all[2].data}));
    // });
      if (!isMounted) {
        console.log("done!");
      }
      isMounted = true;
 
      dispatch({ type: SET_APPLICATION_DATA, days: all[0].data, appointments: all[1].data, interviewers:all[2].data });
  
    });  
  }, []);
  return { state, setDay, bookInterview, cancelInterview }
};

I'd be appreciated for pointing me in the right direction on what I'm doing wrong. Thank you!