How to persist navigating component destination after minimizing the app

I have an app with 3 main destinations which can be accessed by a bottom nav view. Each destination has its own navigation graph.

The problem is that when I minimize and reopen my app, the navigation components reset to the default destination. Why does this happen?

My main activity: (Irrelevant code omitted)

class MainActivity : AppCompatActivity() {

    // List of base host containers
    val fragments = listOf(
            HomeHostFragment(),
            CoursesHostFragment(),
            SearchHostFragment()
    )

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val viewPager: ViewPager = findViewById(R.id.main_activity_pager)
        viewPager.adapter = ViewPagerAdapter()
    }

    inner class ViewPagerAdapter : FragmentPagerAdapter(supportFragmentManager) {
        override fun getItem(position: Int): Fragment =
                fragments[position]

        override fun getCount(): Int =
                fragments.size
    }
}

HomeHostFragment.kt:

class HomeHostFragment : Fragment() {
    private lateinit var navController: NavController

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? =
            inflater.inflate(R.layout.fragment_home_host, container, false)

    override fun onStart() {
        super.onStart()
        navController = requireActivity().findNavController(R.id.nav_host_home)
        navController.setGraph(R.navigation.nav_graph_home)

        NavigationUI.setupWithNavController(toolbar_home, navController)
    }

    fun onBackPressed(): Boolean {
        return navController.navigateUp()
    }
}