UISplitViewController's show details does not seem to replace the detail view

I have an app where there is a UISplitViewController with a navigation controller, itself bound to a UITableViewController displaying a menu to navigate. There is no Detail View Controller being bound, as there is nothing I want to display as a default.

On my physical devices, my code behaves the way I expect it: on iPhone/iPad I can use the UITableViewController and bind my cells to the other UIViewControllers with a Show Detail segue.

On emulator, the same code displays a gray screen in between, as if the Display View Controller were being shown in between, and it does this every single time I perform a Show Detail segue. This is very confusing and I honestly don't know where the bug comes from.

Here is a video that shows the behaviour: https://youtu.be/ygx6kiHux-8

And here are some snippets:

LoginSegue.swift

import UIKit

class LoginSegue: UIStoryboardSegue {
    override func perform() {        
        UIApplication.shared.keyWindow?.rootViewController = self.destination
    }
}

And CustomSplitViewController.swift:

class CustomSplitViewController: UISplitViewController, UISplitViewControllerDelegate {
    
    var user: User? = nil
    

    override func viewDidLoad() {
        super.viewDidLoad()
        self.delegate = self

        if let mainViewController = masterViewController?.topViewController as? MainViewController {
            mainViewController.user = user
        }
        // Do any additional setup after loading the view.
    }
    
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if var signedInViewController = segue.destination as? SignedInViewController {
            signedInViewController.user = self.user
        } else if var mainViewController = segue.destination as? MainViewController {
            mainViewController.user = self.user
        }
    }
    

    // This method is never called, and I don't know why.
    func splitViewController(splitViewController: UISplitViewController, collapseSecondaryViewController secondaryViewController: UIViewController!, ontoPrimaryViewController primaryViewController: UIViewController!) -> Bool{
        return true
      }

}

My MainViewController is the UITableViewController you can see on the video.

Thank you in advance.