UISearchController does not show up in iOS 12

Question

I have an application that shows a list of items retrieved from the API in a UITableView (let's call this as the MasterList). Upon tapping on an item in the MasterList:

1. I push a ViewController (call this as DetailList) onto the Navigation Controller
2. Make a network call to get items as per the selection in MasterList
3. Display the retrieved items in a UITableView in DetailList
4. The network API tells the app whether to show a search-bar or not via a property called showSearchBar.
5. The network API takes about 30ms to come back with the response. But at this point when I try to show the UISearchController in the NavigationItem, it fails to show up.

NOTE: This is a simplified code for easy understanding:

    override func viewDidLoad() {
        super.viewDidLoad()

        //Make a network call to get data
        viewModel.getData()
    }

    //Callback from viewModel when data is received
    func didGetNetworkResponse() {
        configureSearchController()
        //other routines to populate UITableView, etc.
    }

    func configureSearchController() {
        //viewModel decides to show search-bar based on API response 
        //safe to assume the API returns `true`
        guard viewModel.shouldShowSearchBar else { return }

        let searchController = UISearchController(searchResultsController: nil)
        searchController.searchResultsUpdater = self
        searchController.searchBar.autocapitalizationType = .none
        searchController.obscuresBackgroundDuringPresentation = false
        searchController.searchBar.delegate = self
        searchController.searchBar.placeholder = "Search for items"

        navigationItem.searchController = searchController
        navigationItem.hidesSearchBarWhenScrolling = false

        definesPresentationContext = true
        navigationController?.view.setNeedsLayout()
        navigationController?.view.layoutIfNeeded()
    }

Whereas the UISearchController shows up correctly if I try to show it in viewDidLoad() without having to wait for the API response. Also all of the above works perfectly well in iOS 13.x

Questions:

• Am I missing something in the code above
• Is it a known problem with UISearchController not showing up in iOS 11 & 12 when the call to set it is delayed (in this case because of network response)
• Why is the difference of behaviour between iOS 13 and iOS 12

Appreciate your help.