vuex getter does not update on another component depending on timing - vue.js

Question

My app uses
axios to fetch user information from a backend server
vuex to store users
vue-router to navigate on each user's page
In App.vue, the fetch is dispatched
export default {
name: 'app',
components: {
nav00
},
beforeCreate() {
this.$store.dispatch('fetchUsers')
}
}
In store.js, the users is an object with pk (primary key) to user information.
export default new Vuex.Store({
state: {
users: {},
},
getters: {
userCount: state => {
return Object.keys(state.users).length
}
},
mutations: {
SET_USERS(state, users) {
// users should be backend response
console.log(users.length)
users.forEach(u => state.users[u.pk] = u)
actions: {
fetchUsers({commit}) {
Backend.getUsers()
.then(response => {
commit('SET_USERS', response.data)
})
.catch(error => {
console.log("Cannot fetch users: ", error.response)
})
})
Here Backend.getUsers() is an axios call.
In another component which map to /about in the vue-router, it simply displays userCount via the getter.
Now the behavior of the app depends on timing. If I visit / first and wait 2-3 seconds, then go to /about, the userCount is displayed correctly. However, if I visit /about directly, or visit / first and quickly navigate to /about, the userCount is 0. But in the console, it still shows the correct user count (from the log in SET_USERS).
What did I miss here? Shouldn't the store getter see the update in users and render the HTML display again?

Answer 1

Since it's an object Vue can't detect the changes of the properties and it's even less reactive when it comes to computed properties.
Copied from https://vuex.vuejs.org/guide/mutations.html:
When adding new properties to an Object, you should either:
Use Vue.set(obj, 'newProp', 123), or
Replace that Object with a fresh one. For example, using the object spread syntax we can write it like this:
state.obj = { ...state.obj, newProp: 123 }