看起来问题是calendarSlice只为一个日历保存events状态，每次setEvent操作被调度时，它只会替换state.calendar.events，以前存在的任何内容都会被覆盖。
从你的描述中听起来，你需要多个events数组，每个数组对应一个你要查询的日历集合。你可以更新calendarSlice来保存多个日历事件数组。我猜这个集合id nombre足以区分/标识一个唯一日历的事件状态。
建议的重构示例：

const startLoadingEvents = async () => {
  const querySnapshot = await getDocs(collection(db, `${nombre}`));
  const events = [];
  querySnapshot.forEach((doc) => {
    events.push({
      id: doc.id,
      start: doc.data().start.toDate(),
      end: doc.data().end.toDate(),
      title: doc.data().title,
      notes: doc.data().notes,
    });
  });
  dispatch(setEvent({ calendarId: nombre, events }));
};

export const calendarSlice = createSlice({
  name: "calendar",
  initialState: {
    events: {}, // <-- { [nombre]: event[] }
    activeEvent: null,
    isLoadingEvents: true,
  },
  reducers: {
    onSetActiveEvent: (state, { payload }) => {
      state.activeEvent = payload;
    },
    onAddEvent: (state, { payload }) => {
      if (!state.events[payload.calendarId]) {
        state.events[payload.calendarId] = [];
      }
      state.events[payload.calendarId].push(payload.event);
      state.activeEvent = null;
    },
    onUpdateEvent: (state, { payload }) => {
      state.events[payload.calendarId] = state.events[payload.calendarId].map(
        (event) => event.id === payload.event.id) ? payload : event
      );
    },
    onDeleteEvent: (state) => {
      if (state.activeEvent?.calendarId) {
        state.events[state.activeEvent.calendarId] = state.events[state.activeEvent.calendarId]
          .filter((event) => event._id !== state.activeEvent._id);
        state.activeEvent = null;
      }
    },
    setEvent: (state, { payload }) => {
      state.events[payload.calendarId] = payload.events;
    },
    onLoadEvents: (state, { payload }) => {
      state.isLoadingEvents = false;

      (payload.events || []).forEach((event) => {
        const exists = state.events[payload.calendarId]?.some(
          (dbEvent) => dbEvent.id === event.id
        );
        if (!exists) {
          state.events[payload.calendarId].push(event);
        }
      });
    },
  },
});

选择状态时，需要更新以将state.calendar.events作为对象而不是数组使用。

const calendars = useSelector(state => state.calendar.events); // { [nombre]: event[] }

...

calendars[<name of collection>].map(event => { .... })

显然，这一切都没有经过测试，因为我没有访问您的firebase后端，也没有站起来任何运行代码的andbox，但它应该是一个很好的一般开始处理多个日历集合的数据在UI中。