Back button is gone in split view

Question

After embedding the UISplitViewController inside a UITabViewController, back button on navigation bar is turned into a full screen icon:

It's the segue preparation code:

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "showDrugDetails" {
        //foo
        controller.navigationItem.leftBarButtonItem = self.splitViewController?.displayModeButtonItem
        controller.navigationItem.leftItemsSupplementBackButton = 
    }
}

How can I get back the back button? (everything works fine on iPhone)

Edit: Storyboard