MobX: Since strict-mode is enabled, changing (observed) observable values without using an action is not allowed

Your login function is async and you need to use runInAction inside, or handle result in a separate action, or use some other way of handling async actions:

import { runInAction, makeAutoObservable } from "mobx"

class AuthStoreClass {
    authUser = null

    constructor() {
        makeAutoObservable(this)
    }

    login = async (params) => {
        const { data: { data: authUser } } = await loginUser(params)
        
        // Wrap all changes with runInAction
        runInAction(() => {
          this.authUser = authUser
        })

        // or do it in separate function
        this.setUser(authUser)
    }

    // This method will be wrapped into `action` automatically by `makeAutoObservable`
    setUser = (user) => {
        this.authUser = user
    }
}

That is because, citing the docs, every step (“tick”) that updates observables in an asynchronous process should be marked as action. And the code before the first await is in a different “tick” than the code after await.

More about async actions (you can even use generators!): https://mobx.js.org/actions.html#asynchronous-actions

In MobX version 6 actions are enforced by default but you can disable warnings with configure method:

import { configure } from "mobx"

configure({
    enforceActions: "never",
})

But be careful doing it though, the goal of enforceActions is that you don’t forget to wrap event handlers and all mutations in an action. Not doing it might cause extra re-runs of your observers. For example, if you changing two values inside some handler without action then your component might re-render twice instead of once. makeAutoObservable wraps all methods automatically but you still need to handle async methods and Promises manually.