iOS 14 UISplitViewController (sidebar) with triple column sidebar toggle icon behavior

I'm implementing the iOS 14 (iPadOS 14) sidebar (UISplitViewController with TripleColumn) and having strange "sidebar toggle icon" behavior.

In iOS 13 I'm using the tab bar with tabs with "split views" and tab with only "table view" so I need the Triple Column instead of the Double Column to work.

In iOS 13, there is one tab with only a table view, so I set the supplementary view to nil, and hide the view by calling "hide" method implemented in UISplitViewController in iOS 14. (See below for code).

The "sidebar toggle icon" on the upper left is automatically displayed. After clicking the toggle icon, the sidebar hides correctly but an "back button" was created on my secondary view(a UITableViewController embedded in a UINavigationController).

Selecting the back button has no response at all. However, user can still swipe from the left edge of the screen to make sidebar reappear but the "back button" is confusing.
My expected behavior is, after the toggle icon selected in sidebar, display the "sidebar toggle icon" instead of the "back button" in the secondary view. And after pressing the "sidebar toggle icon" in secondary view, the sidebar reappears.

Like the Photos app in iPadOS 14, the toggle button is shown instead of the back button in the secondary view. And clicking the toggle icon will make the sidebar shown again. (but it's a double column split view though, not a triple column split view.) 

SceneDelegate.swift:

Code Block 
func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
    if let windowScene = scene as? UIWindowScene {
        let window = UIWindow(windowScene: windowScene)
        if #available(iOS 14.0, *) {
            let main = UIStoryboard(name: "Main", bundle: nil)
            
            let splitViewController = UISplitViewController(style: .tripleColumn)
            splitViewController.preferredDisplayMode = .twoBesideSecondary
            splitViewController.preferredSplitBehavior = .tile
            splitViewController.setViewController(SideBarViewController(), for: .primary)
            // fall back for compact screen
            splitViewController.setViewController(main.instantiateInitialViewController(), for: .compact)
            window.rootViewController = splitViewController
            self.window = window
            window.makeKeyAndVisible()
        }
    }
}

SideBarViewController.swift:

Code Block 
// if the first tab (dashboard) was selected
private func selectDashboardTab() {
    if #available(iOS 14.0, *) {
        
        let dashboardVC = UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: "DashboardTab") as? UINavigationController
        splitViewController?.preferredPrimaryColumnWidth = 250.0
        splitViewController?.preferredDisplayMode = .twoBesideSecondary
        splitViewController?.preferredSplitBehavior = .tile
        splitViewController?.setViewController(dashboardVC, for: .secondary)
        splitViewController?.setViewController(nil, for: .supplementary)
        splitViewController?.hide(.supplementary)
    }
}